# M4b — ListenBrainz Inbound Similarity Ingest Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Background worker pulls track-track and artist-artist similarity edges from ListenBrainz's public `/explore/*` endpoints, filters to the local library (top-K=20 per source), stores in two new `track_similarity` + `artist_similarity` tables. Hourly tick, 7-day re-fetch cap per row, passive retry via timer.

**Architecture:** New `internal/similarity/` package with a `Worker` goroutine paralleling M4a's scrobble worker. Existing `internal/scrobble/listenbrainz/Client` gets two new methods (`SimilarRecordings`, `SimilarArtists`). New migration adds two symmetric tables with multi-source primary keys + score-DESC indexes for M4c's hot-path query. No frontend changes.

**Tech Stack:** Go 1.23 + sqlc + pgx/v5. No web changes. Runs alongside the M4a scrobble worker.

**Reference:** design spec at `docs/superpowers/specs/2026-04-28-m4b-similarity-design.md`.

---

## File Structure

**New server files:**

| File | Responsibility |
|---|---|
| `internal/db/migrations/0009_similarity.up.sql` + `.down.sql` | `track_similarity` + `artist_similarity` tables (multi-source PK, score-DESC indexes). |
| `internal/db/queries/similarity.sql` | New: `ListPlayedTracksNeedingSimilarity`, `ListPlayedArtistsNeedingSimilarity`, `GetTracksByMBIDs`, `GetArtistsByMBIDs`, `UpsertTrackSimilarity`, `UpsertArtistSimilarity`. |
| `internal/db/dbq/similarity.sql.go` | Generated bindings. |
| `internal/similarity/worker.go` | `Worker` struct, `NewWorker`, `Run` (hourly ticker), `tickOnce` (drains one batch of tracks + one batch of artists). |
| `internal/similarity/worker_test.go` | Pure unit tests (no DB). |
| `internal/similarity/worker_integration_test.go` | Live-DB + httptest LB end-to-end tests. |

**Modified server files:**

| File | Change |
|---|---|
| `internal/scrobble/listenbrainz/client.go` | Add `SimilarRecording` + `SimilarArtist` types, `SimilarRecordings(ctx, mbid, limit)` + `SimilarArtists(ctx, mbid, limit)` methods. Re-uses the existing `Client` struct, error types, and HTTP timeout pattern from M4a. |
| `internal/scrobble/listenbrainz/client_test.go` | 14 new httptest-driven tests (7 per method) covering success path, error mappings, query-param assertions. |
| `cmd/minstrel/main.go` | Construct `similarity.NewWorker(...)` after the M4a scrobble worker; `go similarityWorker.Run(ctx)`. |

**No frontend changes.** M4b is invisible until M4c reads the data.

---

## Task 1: Migration 0009 — schema

**Files:**
- Create: `internal/db/migrations/0009_similarity.up.sql`
- Create: `internal/db/migrations/0009_similarity.down.sql`

- [ ] **Step 1: Write the up migration**

Create `internal/db/migrations/0009_similarity.up.sql`:

```sql
-- M4b: ListenBrainz similarity ingest. Two symmetric tables — tracks and
-- artists — both keyed by (a_id, b_id, source) so the schema reserves room
-- for additional similarity sources (musicbrainz_tag, user_cooccurrence)
-- alongside listenbrainz. M4b only writes 'listenbrainz' rows.
--
-- The (a_id, score DESC) index satisfies M4c's hot path: "for seed S, give
-- me top-N similar tracks/artists descending by score."

CREATE TABLE track_similarity (
    track_a_id   uuid             NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
    track_b_id   uuid             NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
    score        DOUBLE PRECISION NOT NULL,
    source       TEXT             NOT NULL CHECK (source IN ('listenbrainz', 'musicbrainz_tag', 'user_cooccurrence')),
    fetched_at   timestamptz      NOT NULL DEFAULT now(),
    PRIMARY KEY (track_a_id, track_b_id, source),
    CHECK (track_a_id <> track_b_id)
);

CREATE INDEX track_similarity_a_score_idx
    ON track_similarity (track_a_id, score DESC);

CREATE TABLE artist_similarity (
    artist_a_id  uuid             NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
    artist_b_id  uuid             NOT NULL REFERENCES artists(id) ON DELETE CASCADE,
    score        DOUBLE PRECISION NOT NULL,
    source       TEXT             NOT NULL CHECK (source IN ('listenbrainz', 'musicbrainz_tag', 'user_cooccurrence')),
    fetched_at   timestamptz      NOT NULL DEFAULT now(),
    PRIMARY KEY (artist_a_id, artist_b_id, source),
    CHECK (artist_a_id <> artist_b_id)
);

CREATE INDEX artist_similarity_a_score_idx
    ON artist_similarity (artist_a_id, score DESC);
```

- [ ] **Step 2: Write the down migration**

Create `internal/db/migrations/0009_similarity.down.sql`:

```sql
DROP TABLE IF EXISTS artist_similarity;
DROP TABLE IF EXISTS track_similarity;
```

- [ ] **Step 3: Verify the migration applies cleanly**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && MINSTREL_TEST_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' go test -race ./internal/db/`

Expected: PASS. The migration test should now find version 9.

- [ ] **Step 4: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/db/migrations/0009_similarity.up.sql internal/db/migrations/0009_similarity.down.sql
git commit -m "feat(db): add migration 0009 for similarity (track_similarity + artist_similarity)"
```

---

## Task 2: sqlc queries

**Files:**
- Create: `internal/db/queries/similarity.sql`
- Generated: `internal/db/dbq/similarity.sql.go`

- [ ] **Step 1: Create the queries file**

Create `internal/db/queries/similarity.sql`:

```sql
-- name: ListPlayedTracksNeedingSimilarity :many
-- Tracks with at least one play, an MBID, and no fresh listenbrainz row
-- (no row at all, OR fetched_at older than 7 days). Used by the worker
-- to find work each tick. Bounded by $1.
SELECT DISTINCT t.id, t.mbid
FROM tracks t
JOIN play_events pe ON pe.track_id = t.id
WHERE t.mbid IS NOT NULL
  AND NOT EXISTS (
      SELECT 1 FROM track_similarity ts
      WHERE ts.track_a_id = t.id
        AND ts.source = 'listenbrainz'
        AND ts.fetched_at > now() - interval '7 days'
  )
ORDER BY t.id
LIMIT $1;

-- name: ListPlayedArtistsNeedingSimilarity :many
SELECT DISTINCT ar.id, ar.mbid
FROM artists ar
JOIN tracks t ON t.artist_id = ar.id
JOIN play_events pe ON pe.track_id = t.id
WHERE ar.mbid IS NOT NULL
  AND NOT EXISTS (
      SELECT 1 FROM artist_similarity asim
      WHERE asim.artist_a_id = ar.id
        AND asim.source = 'listenbrainz'
        AND asim.fetched_at > now() - interval '7 days'
  )
ORDER BY ar.id
LIMIT $1;

-- name: GetTracksByMBIDs :many
-- Bulk in-library lookup: maps a slice of MBIDs back to local track IDs.
-- Used by the worker to filter LB-returned MBIDs to those actually in the
-- user's library.
SELECT id, mbid FROM tracks WHERE mbid = ANY($1::text[]);

-- name: GetArtistsByMBIDs :many
SELECT id, mbid FROM artists WHERE mbid = ANY($1::text[]);

-- name: UpsertTrackSimilarity :exec
-- Refresh-in-place: if (a, b, source) already exists, update score and
-- fetched_at. Idempotent.
INSERT INTO track_similarity (track_a_id, track_b_id, score, source, fetched_at)
VALUES ($1, $2, $3, 'listenbrainz', now())
ON CONFLICT (track_a_id, track_b_id, source)
DO UPDATE SET score = EXCLUDED.score, fetched_at = EXCLUDED.fetched_at;

-- name: UpsertArtistSimilarity :exec
INSERT INTO artist_similarity (artist_a_id, artist_b_id, score, source, fetched_at)
VALUES ($1, $2, $3, 'listenbrainz', now())
ON CONFLICT (artist_a_id, artist_b_id, source)
DO UPDATE SET score = EXCLUDED.score, fetched_at = EXCLUDED.fetched_at;
```

- [ ] **Step 2: Run sqlc generate**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && make generate`

Expected: dockerized sqlc 1.27.0 regenerates `internal/db/dbq/similarity.sql.go` with the 6 new methods. Other `*.sql.go` files may receive trivial version-comment updates from sqlc — stage them all.

- [ ] **Step 3: Verify compile**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go build ./...`

Expected: clean. No callers yet — generated bindings just need to compile.

- [ ] **Step 4: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/db/queries/similarity.sql internal/db/dbq/
git commit -m "feat(db): add similarity sqlc queries (list-needing, get-by-mbids, upsert)"
```

---

## Task 3: LB client `SimilarRecordings` method

**Files:**
- Modify: `internal/scrobble/listenbrainz/client.go` (append types + method)
- Modify: `internal/scrobble/listenbrainz/client_test.go` (append tests)

The existing `Client` already has `SubmitListens` from M4a with `BaseURL`, `HTTP`, and the typed errors `ErrAuth`/`ErrPermanent`/`ErrTransient`/`*RetryAfterError`. We add a sibling read-only method.

LB algorithm parameter: hardcode `session_based_days_7500_session_30_contribution_5_threshold_15_limit_100_filter_True_skip_30` — this is the documented default at the time of writing. If LB changes its default at implementation time, swap to whatever's current per their `/explore/similar-recordings/` docs.

- [ ] **Step 1: Write the failing tests**

Append to `internal/scrobble/listenbrainz/client_test.go`:

```go
func TestClient_SimilarRecordings_Success(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		// Body shape from LB's API docs (simplified for tests).
		_, _ = w.Write([]byte(`[
			{"recording_mbid": "11111111-1111-1111-1111-111111111111", "score": 0.95},
			{"recording_mbid": "22222222-2222-2222-2222-222222222222", "score": 0.80}
		]`))
	})
	defer srv.Close()
	out, err := c.SimilarRecordings(context.Background(), "aaaa", 100)
	if err != nil {
		t.Fatalf("err: %v", err)
	}
	if len(out) != 2 {
		t.Fatalf("got %d results, want 2", len(out))
	}
	if out[0].MBID != "11111111-1111-1111-1111-111111111111" || out[0].Score != 0.95 {
		t.Errorf("first result wrong: %+v", out[0])
	}
}

func TestClient_SimilarRecordings_AlgorithmParamSet(t *testing.T) {
	var seenURL string
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		seenURL = r.URL.String()
		_, _ = w.Write([]byte(`[]`))
	})
	defer srv.Close()
	_, _ = c.SimilarRecordings(context.Background(), "abc", 50)
	if !strings.Contains(seenURL, "algorithm=") {
		t.Errorf("URL missing algorithm param: %q", seenURL)
	}
}

func TestClient_SimilarRecordings_LimitParamSet(t *testing.T) {
	var seenURL string
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		seenURL = r.URL.String()
		_, _ = w.Write([]byte(`[]`))
	})
	defer srv.Close()
	_, _ = c.SimilarRecordings(context.Background(), "abc", 50)
	if !strings.Contains(seenURL, "count=50") && !strings.Contains(seenURL, "limit=50") {
		t.Errorf("URL missing count/limit param: %q", seenURL)
	}
}

func TestClient_SimilarRecordings_401_ReturnsErrAuth(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusUnauthorized)
	})
	defer srv.Close()
	_, err := c.SimilarRecordings(context.Background(), "abc", 100)
	if !errors.Is(err, ErrAuth) {
		t.Errorf("err = %v, want ErrAuth", err)
	}
}

func TestClient_SimilarRecordings_400_ReturnsErrPermanent(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusBadRequest)
	})
	defer srv.Close()
	_, err := c.SimilarRecordings(context.Background(), "abc", 100)
	if !errors.Is(err, ErrPermanent) {
		t.Errorf("err = %v, want ErrPermanent", err)
	}
}

func TestClient_SimilarRecordings_503_ReturnsErrTransient(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusServiceUnavailable)
	})
	defer srv.Close()
	_, err := c.SimilarRecordings(context.Background(), "abc", 100)
	if !errors.Is(err, ErrTransient) {
		t.Errorf("err = %v, want ErrTransient", err)
	}
}

func TestClient_SimilarRecordings_429_ReturnsRetryAfter(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Retry-After", "120")
		w.WriteHeader(http.StatusTooManyRequests)
	})
	defer srv.Close()
	_, err := c.SimilarRecordings(context.Background(), "abc", 100)
	var ra *RetryAfterError
	if !errors.As(err, &ra) {
		t.Fatalf("err = %v, want *RetryAfterError", err)
	}
	if ra.Wait != 120*time.Second {
		t.Errorf("Wait = %v, want 120s", ra.Wait)
	}
}
```

Make sure `strings` is in the existing import block at the top of the file (it almost certainly already is from M4a — if not, add it).

- [ ] **Step 2: Run tests to verify they fail**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/scrobble/listenbrainz/ -run SimilarRecordings -v`

Expected: FAIL with "undefined: SimilarRecording" / "undefined: SimilarRecordings".

- [ ] **Step 3: Implement the method**

Append to `internal/scrobble/listenbrainz/client.go` (no new imports needed beyond what M4a already has — `bytes`, `context`, `encoding/json`, `errors`, `fmt`, `net/http`, `strconv`, `time`):

```go
// LB algorithm parameter for /explore/similar-recordings/. Hardcoded for v1
// — matches LB's documented default. If LB changes the default, swap here.
const lbSimilarRecordingsAlgorithm = "session_based_days_7500_session_30_contribution_5_threshold_15_limit_100_filter_True_skip_30"

// LB algorithm parameter for /explore/similar-artists/.
const lbSimilarArtistsAlgorithm = "session_based_days_7500_session_30_contribution_5_threshold_15_limit_100_filter_True_skip_30"

// SimilarRecording is one entry in the /explore/similar-recordings response.
type SimilarRecording struct {
	MBID  string  `json:"recording_mbid"`
	Score float64 `json:"score"`
}

// SimilarRecordings fetches up to `limit` similar recordings for the given
// recording MBID. Public endpoint — no token required. Returns the same
// typed errors as SubmitListens.
func (c *Client) SimilarRecordings(ctx context.Context, mbid string, limit int) ([]SimilarRecording, error) {
	base := c.BaseURL
	if base == "" {
		base = defaultBaseURL
	}
	url := fmt.Sprintf("%s/1/explore/similar-recordings/%s?algorithm=%s&count=%d",
		base, mbid, lbSimilarRecordingsAlgorithm, limit)
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, fmt.Errorf("listenbrainz: build similar-recordings request: %w", err)
	}

	httpClient := c.HTTP
	if httpClient == nil {
		httpClient = http.DefaultClient
	}
	resp, err := httpClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrTransient, err)
	}
	defer func() { _ = resp.Body.Close() }()

	switch {
	case resp.StatusCode >= 200 && resp.StatusCode < 300:
		var out []SimilarRecording
		if derr := json.NewDecoder(resp.Body).Decode(&out); derr != nil {
			return nil, fmt.Errorf("%w: decode similar-recordings: %v", ErrTransient, derr)
		}
		return out, nil
	case resp.StatusCode == http.StatusUnauthorized:
		return nil, ErrAuth
	case resp.StatusCode == http.StatusTooManyRequests:
		return nil, &RetryAfterError{Wait: parseRetryAfter(resp.Header.Get("Retry-After"))}
	case resp.StatusCode >= 500:
		return nil, fmt.Errorf("%w: status %d", ErrTransient, resp.StatusCode)
	default:
		return nil, fmt.Errorf("%w: status %d", ErrPermanent, resp.StatusCode)
	}
}
```

- [ ] **Step 4: Run tests to verify they pass**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/scrobble/listenbrainz/ -run SimilarRecordings -v`

Expected: PASS for all 7 new tests.

- [ ] **Step 5: Verify lint clean**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./internal/scrobble/...`

Expected: clean.

- [ ] **Step 6: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/scrobble/listenbrainz/client.go internal/scrobble/listenbrainz/client_test.go
git commit -m "feat(listenbrainz): add SimilarRecordings client method"
```

---

## Task 4: LB client `SimilarArtists` method

Symmetric to Task 3. The artist endpoint has the same shape; the response key is `artist_mbid` instead of `recording_mbid`.

**Files:**
- Modify: `internal/scrobble/listenbrainz/client.go` (append)
- Modify: `internal/scrobble/listenbrainz/client_test.go` (append)

- [ ] **Step 1: Write the failing tests**

Append to `internal/scrobble/listenbrainz/client_test.go`:

```go
func TestClient_SimilarArtists_Success(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		_, _ = w.Write([]byte(`[
			{"artist_mbid": "33333333-3333-3333-3333-333333333333", "score": 0.91},
			{"artist_mbid": "44444444-4444-4444-4444-444444444444", "score": 0.72}
		]`))
	})
	defer srv.Close()
	out, err := c.SimilarArtists(context.Background(), "aaaa", 100)
	if err != nil {
		t.Fatalf("err: %v", err)
	}
	if len(out) != 2 {
		t.Fatalf("got %d, want 2", len(out))
	}
	if out[0].MBID != "33333333-3333-3333-3333-333333333333" || out[0].Score != 0.91 {
		t.Errorf("first result wrong: %+v", out[0])
	}
}

func TestClient_SimilarArtists_AlgorithmParamSet(t *testing.T) {
	var seenURL string
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		seenURL = r.URL.String()
		_, _ = w.Write([]byte(`[]`))
	})
	defer srv.Close()
	_, _ = c.SimilarArtists(context.Background(), "abc", 50)
	if !strings.Contains(seenURL, "algorithm=") {
		t.Errorf("URL missing algorithm param: %q", seenURL)
	}
}

func TestClient_SimilarArtists_LimitParamSet(t *testing.T) {
	var seenURL string
	c, srv := newTestClient(func(w http.ResponseWriter, r *http.Request) {
		seenURL = r.URL.String()
		_, _ = w.Write([]byte(`[]`))
	})
	defer srv.Close()
	_, _ = c.SimilarArtists(context.Background(), "abc", 50)
	if !strings.Contains(seenURL, "count=50") && !strings.Contains(seenURL, "limit=50") {
		t.Errorf("URL missing count/limit param: %q", seenURL)
	}
}

func TestClient_SimilarArtists_401_ReturnsErrAuth(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusUnauthorized)
	})
	defer srv.Close()
	_, err := c.SimilarArtists(context.Background(), "abc", 100)
	if !errors.Is(err, ErrAuth) {
		t.Errorf("err = %v, want ErrAuth", err)
	}
}

func TestClient_SimilarArtists_400_ReturnsErrPermanent(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusBadRequest)
	})
	defer srv.Close()
	_, err := c.SimilarArtists(context.Background(), "abc", 100)
	if !errors.Is(err, ErrPermanent) {
		t.Errorf("err = %v, want ErrPermanent", err)
	}
}

func TestClient_SimilarArtists_503_ReturnsErrTransient(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.WriteHeader(http.StatusServiceUnavailable)
	})
	defer srv.Close()
	_, err := c.SimilarArtists(context.Background(), "abc", 100)
	if !errors.Is(err, ErrTransient) {
		t.Errorf("err = %v, want ErrTransient", err)
	}
}

func TestClient_SimilarArtists_429_ReturnsRetryAfter(t *testing.T) {
	c, srv := newTestClient(func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Retry-After", "60")
		w.WriteHeader(http.StatusTooManyRequests)
	})
	defer srv.Close()
	_, err := c.SimilarArtists(context.Background(), "abc", 100)
	var ra *RetryAfterError
	if !errors.As(err, &ra) {
		t.Fatalf("err = %v, want *RetryAfterError", err)
	}
	if ra.Wait != 60*time.Second {
		t.Errorf("Wait = %v, want 60s", ra.Wait)
	}
}
```

- [ ] **Step 2: Run tests to verify they fail**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/scrobble/listenbrainz/ -run SimilarArtists -v`

Expected: FAIL with "undefined: SimilarArtists".

- [ ] **Step 3: Implement the method**

Append to `internal/scrobble/listenbrainz/client.go`:

```go
// SimilarArtist is one entry in the /explore/similar-artists response.
type SimilarArtist struct {
	MBID  string  `json:"artist_mbid"`
	Score float64 `json:"score"`
}

// SimilarArtists fetches up to `limit` similar artists for the given
// artist MBID. Public endpoint — no token required. Returns the same
// typed errors as SubmitListens.
func (c *Client) SimilarArtists(ctx context.Context, mbid string, limit int) ([]SimilarArtist, error) {
	base := c.BaseURL
	if base == "" {
		base = defaultBaseURL
	}
	url := fmt.Sprintf("%s/1/explore/similar-artists/%s?algorithm=%s&count=%d",
		base, mbid, lbSimilarArtistsAlgorithm, limit)
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, fmt.Errorf("listenbrainz: build similar-artists request: %w", err)
	}

	httpClient := c.HTTP
	if httpClient == nil {
		httpClient = http.DefaultClient
	}
	resp, err := httpClient.Do(req)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrTransient, err)
	}
	defer func() { _ = resp.Body.Close() }()

	switch {
	case resp.StatusCode >= 200 && resp.StatusCode < 300:
		var out []SimilarArtist
		if derr := json.NewDecoder(resp.Body).Decode(&out); derr != nil {
			return nil, fmt.Errorf("%w: decode similar-artists: %v", ErrTransient, derr)
		}
		return out, nil
	case resp.StatusCode == http.StatusUnauthorized:
		return nil, ErrAuth
	case resp.StatusCode == http.StatusTooManyRequests:
		return nil, &RetryAfterError{Wait: parseRetryAfter(resp.Header.Get("Retry-After"))}
	case resp.StatusCode >= 500:
		return nil, fmt.Errorf("%w: status %d", ErrTransient, resp.StatusCode)
	default:
		return nil, fmt.Errorf("%w: status %d", ErrPermanent, resp.StatusCode)
	}
}
```

- [ ] **Step 4: Run tests to verify they pass**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/scrobble/listenbrainz/ -run SimilarArtists -v`

Expected: PASS for all 7 new tests.

- [ ] **Step 5: Verify lint clean**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./internal/scrobble/...`

Expected: clean.

- [ ] **Step 6: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/scrobble/listenbrainz/client.go internal/scrobble/listenbrainz/client_test.go
git commit -m "feat(listenbrainz): add SimilarArtists client method"
```

---

## Task 5: Worker skeleton + pure unit tests

The Worker package gets skeleton types + `NewWorker` constructor first. Integration tests in Task 6 exercise the full pipeline.

**Files:**
- Create: `internal/similarity/worker.go`
- Create: `internal/similarity/worker_test.go`

- [ ] **Step 1: Write the failing tests**

Create `internal/similarity/worker_test.go`:

```go
package similarity

import (
	"io"
	"log/slog"
	"testing"
	"time"
)

func TestNewWorker_DefaultsMatchSpec(t *testing.T) {
	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
	w := NewWorker(nil, nil, logger)
	if w.tick != 1*time.Hour {
		t.Errorf("tick = %v, want 1h", w.tick)
	}
	if w.batch != 5 {
		t.Errorf("batch = %d, want 5", w.batch)
	}
	if w.topK != 20 {
		t.Errorf("topK = %d, want 20", w.topK)
	}
}
```

- [ ] **Step 2: Run tests to verify they fail**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/similarity/ -v`

Expected: FAIL — package doesn't exist yet.

- [ ] **Step 3: Write the worker skeleton**

Create `internal/similarity/worker.go`:

```go
// Package similarity owns the inbound ListenBrainz similarity ingest
// pipeline. A periodic worker queries LB's /explore/similar-recordings
// and /explore/similar-artists endpoints for tracks the user has played,
// filters returned MBIDs to the local library, and stores the top-K
// edges in track_similarity / artist_similarity for M4c's radio
// candidate-pool builder.
package similarity

import (
	"context"
	"log/slog"
	"time"

	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/scrobble/listenbrainz"
)

// Worker drains played-tracks-and-artists needing similarity and POSTs
// the results into track_similarity / artist_similarity. Failures are
// passively retried via the timer (no durable queue table — losing one
// tick's worth of refresh attempts is "1 hour of staleness," fine).
type Worker struct {
	pool   *pgxpool.Pool
	client *listenbrainz.Client
	logger *slog.Logger
	tick   time.Duration
	batch  int32
	topK   int
}

// NewWorker constructs a worker with production defaults: 1h tick,
// batch=5, topK=20.
func NewWorker(pool *pgxpool.Pool, client *listenbrainz.Client, logger *slog.Logger) *Worker {
	return &Worker{
		pool:   pool,
		client: client,
		logger: logger,
		tick:   1 * time.Hour,
		batch:  5,
		topK:   20,
	}
}

// Run blocks until ctx is cancelled, ticking every w.tick.
func (w *Worker) Run(ctx context.Context) {
	t := time.NewTicker(w.tick)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			if err := w.tickOnce(ctx); err != nil {
				w.logger.Error("similarity: tick failed", "err", err)
			}
		}
	}
}

// tickOnce drains one batch of tracks and one batch of artists. Stub —
// implementation lands in Task 6 along with the integration tests that
// drive it.
func (w *Worker) tickOnce(ctx context.Context) error {
	return nil
}
```

- [ ] **Step 4: Run tests to verify they pass**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test ./internal/similarity/ -v`

Expected: PASS for the constructor test.

- [ ] **Step 5: Verify lint clean**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./internal/similarity/...`

Expected: clean.

- [ ] **Step 6: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/similarity/worker.go internal/similarity/worker_test.go
git commit -m "feat(similarity): add Worker skeleton with constructor + Run loop"
```

---

## Task 6: Worker tickOnce — full pipeline (integration)

Full implementation of the worker drain path with comprehensive integration tests.

**Files:**
- Modify: `internal/similarity/worker.go` (replace `tickOnce` stub with real impl)
- Create: `internal/similarity/worker_integration_test.go`

The test file follows the M4a fixture pattern: a test pool helper that runs migrations, truncates known tables, and returns a `*pgxpool.Pool` + `*dbq.Queries`. Since the M4a `internal/scrobble` package already has a similar helper in `queue_test.go`, copy that shape.

- [ ] **Step 1: Write the failing integration tests**

Create `internal/similarity/worker_integration_test.go`:

```go
package similarity

import (
	"context"
	"encoding/json"
	"io"
	"log/slog"
	"net/http"
	"net/http/httptest"
	"os"
	"strings"
	"sync/atomic"
	"testing"
	"time"

	"github.com/jackc/pgx/v5/pgtype"
	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/scrobble/listenbrainz"
)

func testPool(t *testing.T) (*pgxpool.Pool, *dbq.Queries) {
	t.Helper()
	if testing.Short() {
		t.Skip("skipping in -short mode")
	}
	dsn := os.Getenv("MINSTREL_TEST_DATABASE_URL")
	if dsn == "" {
		t.Skip("MINSTREL_TEST_DATABASE_URL not set")
	}
	if err := db.Migrate(dsn, slog.New(slog.NewTextHandler(io.Discard, nil))); err != nil {
		t.Fatalf("migrate: %v", err)
	}
	pool, err := pgxpool.New(context.Background(), dsn)
	if err != nil {
		t.Fatalf("pool: %v", err)
	}
	t.Cleanup(pool.Close)
	if _, err := pool.Exec(context.Background(),
		"TRUNCATE artist_similarity, track_similarity, scrobble_queue, play_events, skip_events, play_sessions, sessions, users, tracks, albums, artists RESTART IDENTITY CASCADE"); err != nil {
		t.Fatalf("truncate: %v", err)
	}
	return pool, dbq.New(pool)
}

type fixture struct {
	pool   *pgxpool.Pool
	q      *dbq.Queries
	user   pgtype.UUID
	artist dbq.Artist
	album  dbq.Album
}

// newFixture creates a user, one artist (with MBID), and one album.
// Tests add tracks via seedTrack which optionally takes an MBID.
func newFixture(t *testing.T) fixture {
	t.Helper()
	pool, q := testPool(t)
	ctx := context.Background()
	u, err := q.CreateUser(ctx, dbq.CreateUserParams{
		Username: "tester", PasswordHash: "x", ApiToken: "x", IsAdmin: false,
	})
	if err != nil {
		t.Fatalf("user: %v", err)
	}
	artistMbid := "aaaaaaaa-1111-1111-1111-111111111111"
	a, err := q.UpsertArtist(ctx, dbq.UpsertArtistParams{
		Name: "X", SortName: "X", Mbid: &artistMbid,
	})
	if err != nil {
		t.Fatalf("artist: %v", err)
	}
	al, err := q.UpsertAlbum(ctx, dbq.UpsertAlbumParams{
		Title: "X", SortTitle: "X", ArtistID: a.ID,
	})
	if err != nil {
		t.Fatalf("album: %v", err)
	}
	return fixture{pool: pool, q: q, user: u.ID, artist: a, album: al}
}

func seedTrack(t *testing.T, f fixture, title string, mbid *string) dbq.Track {
	t.Helper()
	tr, err := f.q.UpsertTrack(context.Background(), dbq.UpsertTrackParams{
		Title:    title,
		AlbumID:  f.album.ID,
		ArtistID: f.artist.ID,
		FilePath: "/tmp/" + title + ".flac",
		DurationMs: 200_000,
		Mbid:     mbid,
	})
	if err != nil {
		t.Fatalf("track: %v", err)
	}
	return tr
}

// markPlayed inserts a closed play_event so the track qualifies as "played"
// for the worker's selection query.
func markPlayed(t *testing.T, f fixture, trackID pgtype.UUID) {
	t.Helper()
	var sessionID pgtype.UUID
	if err := f.pool.QueryRow(context.Background(),
		`INSERT INTO play_sessions (user_id, started_at, last_event_at, client_id)
		 VALUES ($1, now() - interval '5 minutes', now(), 'test') RETURNING id`,
		f.user).Scan(&sessionID); err != nil {
		t.Fatalf("session: %v", err)
	}
	if _, err := f.pool.Exec(context.Background(),
		`INSERT INTO play_events (user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped)
		 VALUES ($1, $2, $3, now() - interval '1 minute', now(), 250000, 0.83, false)`,
		f.user, trackID, sessionID); err != nil {
		t.Fatalf("play_event: %v", err)
	}
}

func newTestWorker(f fixture, lbBaseURL string) *Worker {
	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
	return &Worker{
		pool:   f.pool,
		client: &listenbrainz.Client{BaseURL: lbBaseURL, HTTP: http.DefaultClient},
		logger: logger,
		tick:   1 * time.Hour,
		batch:  5,
		topK:   20,
	}
}

// stubLB returns an httptest server that responds to similar-recordings and
// similar-artists with the given JSON payloads. Use empty string to simulate
// "endpoint returned []" (LB knows the MBID but has no neighbors).
func stubLB(recordingsBody, artistsBody string, status int) *httptest.Server {
	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if status != 0 && status != http.StatusOK {
			w.WriteHeader(status)
			return
		}
		if strings.Contains(r.URL.Path, "/similar-recordings/") {
			_, _ = w.Write([]byte(recordingsBody))
			return
		}
		if strings.Contains(r.URL.Path, "/similar-artists/") {
			_, _ = w.Write([]byte(artistsBody))
			return
		}
		w.WriteHeader(http.StatusNotFound)
	}))
}

func countTrackSim(t *testing.T, f fixture, a pgtype.UUID) int {
	t.Helper()
	var n int
	if err := f.pool.QueryRow(context.Background(),
		`SELECT count(*) FROM track_similarity WHERE track_a_id = $1`, a).Scan(&n); err != nil {
		t.Fatalf("count: %v", err)
	}
	return n
}

func countArtistSim(t *testing.T, f fixture, a pgtype.UUID) int {
	t.Helper()
	var n int
	if err := f.pool.QueryRow(context.Background(),
		`SELECT count(*) FROM artist_similarity WHERE artist_a_id = $1`, a).Scan(&n); err != nil {
		t.Fatalf("count: %v", err)
	}
	return n
}

func TestTickOnce_NoPlayedTracks_NoOp(t *testing.T) {
	f := newFixture(t)
	srv := stubLB(`[]`, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	// no rows inserted anywhere
	var n int
	_ = f.pool.QueryRow(context.Background(), `SELECT count(*) FROM track_similarity`).Scan(&n)
	if n != 0 {
		t.Errorf("track_similarity rows = %d, want 0", n)
	}
	_ = f.pool.QueryRow(context.Background(), `SELECT count(*) FROM artist_similarity`).Scan(&n)
	if n != 0 {
		t.Errorf("artist_similarity rows = %d, want 0", n)
	}
}

func TestTickOnce_MapsLBResponseToLocalLibrary(t *testing.T) {
	f := newFixture(t)
	mbidA := "11111111-1111-1111-1111-111111111111"
	mbidB := "22222222-2222-2222-2222-222222222222"
	mbidC := "99999999-9999-9999-9999-999999999999" // NOT in library
	trackA := seedTrack(t, f, "A", &mbidA)
	trackB := seedTrack(t, f, "B", &mbidB)
	markPlayed(t, f, trackA.ID)
	// LB returns three results when queried for trackA — two in library, one not.
	body := `[
		{"recording_mbid": "` + mbidB + `", "score": 0.9},
		{"recording_mbid": "` + mbidC + `", "score": 0.7}
	]`
	_ = trackB
	srv := stubLB(body, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	if got := countTrackSim(t, f, trackA.ID); got != 1 {
		t.Errorf("track_similarity rows = %d, want 1 (only mbidB is in-library)", got)
	}
}

func TestTickOnce_TopKEnforced(t *testing.T) {
	f := newFixture(t)
	mbidSeed := "11111111-1111-1111-1111-111111111111"
	seed := seedTrack(t, f, "Seed", &mbidSeed)
	markPlayed(t, f, seed.ID)
	// Build 25 tracks with MBIDs in library, plus the LB response listing all 25.
	type lbRow struct {
		MBID  string  `json:"recording_mbid"`
		Score float64 `json:"score"`
	}
	rows := make([]lbRow, 0, 25)
	for i := 0; i < 25; i++ {
		mbid := "20000000-0000-0000-0000-" + leftPad(i+1, 12)
		_ = seedTrack(t, f, "T"+leftPad(i+1, 2), &mbid)
		rows = append(rows, lbRow{MBID: mbid, Score: 1.0 - 0.01*float64(i)})
	}
	body, _ := json.Marshal(rows)
	srv := stubLB(string(body), `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	if got := countTrackSim(t, f, seed.ID); got > 20 {
		t.Errorf("top-K not enforced: got %d, want ≤ 20", got)
	}
	if got := countTrackSim(t, f, seed.ID); got != 20 {
		t.Errorf("got %d rows, want exactly 20 (all 25 in-library; top 20 by score)", got)
	}
}

func TestTickOnce_RespectsSevenDayCap(t *testing.T) {
	f := newFixture(t)
	mbid := "11111111-1111-1111-1111-111111111111"
	trackA := seedTrack(t, f, "A", &mbid)
	markPlayed(t, f, trackA.ID)
	// Manually insert a fresh row so the worker should skip this track.
	otherMbid := "55555555-5555-5555-5555-555555555555"
	other := seedTrack(t, f, "Other", &otherMbid)
	if _, err := f.pool.Exec(context.Background(),
		`INSERT INTO track_similarity (track_a_id, track_b_id, score, source, fetched_at)
		 VALUES ($1, $2, 0.5, 'listenbrainz', now())`, trackA.ID, other.ID); err != nil {
		t.Fatalf("seed sim: %v", err)
	}
	// LB stub would return more rows if called; if the worker hits LB the
	// test fails (different count after).
	beforeCount := countTrackSim(t, f, trackA.ID)
	srv := stubLB(`[{"recording_mbid":"`+otherMbid+`","score":0.99}]`, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	_ = w.tickOnce(context.Background())
	if got := countTrackSim(t, f, trackA.ID); got != beforeCount {
		t.Errorf("worker re-queried fresh track: before=%d after=%d", beforeCount, got)
	}
}

func TestTickOnce_RefreshesStaleRow(t *testing.T) {
	f := newFixture(t)
	mbid := "11111111-1111-1111-1111-111111111111"
	trackA := seedTrack(t, f, "A", &mbid)
	markPlayed(t, f, trackA.ID)
	otherMbid := "55555555-5555-5555-5555-555555555555"
	other := seedTrack(t, f, "Other", &otherMbid)
	// Insert a stale row from 8 days ago with score 0.5.
	if _, err := f.pool.Exec(context.Background(),
		`INSERT INTO track_similarity (track_a_id, track_b_id, score, source, fetched_at)
		 VALUES ($1, $2, 0.5, 'listenbrainz', now() - interval '8 days')`, trackA.ID, other.ID); err != nil {
		t.Fatalf("seed sim: %v", err)
	}
	srv := stubLB(`[{"recording_mbid":"`+otherMbid+`","score":0.95}]`, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	var score float64
	var fetchedAt time.Time
	_ = f.pool.QueryRow(context.Background(),
		`SELECT score, fetched_at FROM track_similarity WHERE track_a_id = $1 AND track_b_id = $2 AND source = 'listenbrainz'`,
		trackA.ID, other.ID).Scan(&score, &fetchedAt)
	if score != 0.95 {
		t.Errorf("score = %v, want 0.95 (refreshed)", score)
	}
	if time.Since(fetchedAt) > time.Minute {
		t.Errorf("fetched_at not bumped: %v ago", time.Since(fetchedAt))
	}
}

func TestTickOnce_429AbortsTick(t *testing.T) {
	f := newFixture(t)
	mbid := "11111111-1111-1111-1111-111111111111"
	trackA := seedTrack(t, f, "A", &mbid)
	markPlayed(t, f, trackA.ID)
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Retry-After", "60")
		w.WriteHeader(http.StatusTooManyRequests)
	}))
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	_ = w.tickOnce(context.Background())
	// no rows updated; fetched_at should not be set anywhere
	var n int
	_ = f.pool.QueryRow(context.Background(), `SELECT count(*) FROM track_similarity WHERE track_a_id = $1`, trackA.ID).Scan(&n)
	if n != 0 {
		t.Errorf("rows on 429 = %d, want 0 (tick aborted before any inserts)", n)
	}
}

func TestTickOnce_TransientErrorSkipsTrack(t *testing.T) {
	f := newFixture(t)
	mbidA := "11111111-1111-1111-1111-111111111111"
	mbidB := "22222222-2222-2222-2222-222222222222"
	otherMbid := "55555555-5555-5555-5555-555555555555"
	trackA := seedTrack(t, f, "A", &mbidA)
	trackB := seedTrack(t, f, "B", &mbidB)
	other := seedTrack(t, f, "Other", &otherMbid)
	markPlayed(t, f, trackA.ID)
	markPlayed(t, f, trackB.ID)
	// Stub: 503 for the first MBID we see, success body for the second.
	var seen atomic.Int32
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !strings.Contains(r.URL.Path, "/similar-recordings/") {
			_, _ = w.Write([]byte(`[]`))
			return
		}
		n := seen.Add(1)
		if n == 1 {
			w.WriteHeader(http.StatusServiceUnavailable)
			return
		}
		_, _ = w.Write([]byte(`[{"recording_mbid":"` + otherMbid + `","score":0.7}]`))
	}))
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	_ = w.tickOnce(context.Background())
	_ = other // referenced via UPSERT
	// Exactly one of trackA/trackB should have a row; the other (whichever
	// hit 503) should be empty.
	a := countTrackSim(t, f, trackA.ID)
	b := countTrackSim(t, f, trackB.ID)
	if a+b != 1 {
		t.Errorf("expected exactly one track to succeed: a=%d b=%d", a, b)
	}
}

func TestTickOnce_FiltersInLibrary(t *testing.T) {
	f := newFixture(t)
	mbid := "11111111-1111-1111-1111-111111111111"
	trackA := seedTrack(t, f, "A", &mbid)
	markPlayed(t, f, trackA.ID)
	// LB returns 3 MBIDs none of which are in our library.
	body := `[
		{"recording_mbid":"99999999-9999-9999-9999-999999999991","score":0.9},
		{"recording_mbid":"99999999-9999-9999-9999-999999999992","score":0.8},
		{"recording_mbid":"99999999-9999-9999-9999-999999999993","score":0.7}
	]`
	srv := stubLB(body, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	if got := countTrackSim(t, f, trackA.ID); got != 0 {
		t.Errorf("filtered out-of-library: got %d, want 0", got)
	}
}

func TestTickOnce_ArtistPassMirrors(t *testing.T) {
	f := newFixture(t)
	mbid := "11111111-1111-1111-1111-111111111111"
	trackA := seedTrack(t, f, "A", &mbid)
	markPlayed(t, f, trackA.ID)
	// Seed a SECOND artist with MBID; LB returns it as similar to f.artist.
	bMbid := "bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb"
	otherArtist, err := f.q.UpsertArtist(context.Background(), dbq.UpsertArtistParams{
		Name: "Other", SortName: "Other", Mbid: &bMbid,
	})
	if err != nil {
		t.Fatalf("artist: %v", err)
	}
	body := `[{"artist_mbid":"` + bMbid + `","score":0.85}]`
	srv := stubLB(`[]`, body, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	_ = otherArtist
	if got := countArtistSim(t, f, f.artist.ID); got != 1 {
		t.Errorf("artist_similarity rows = %d, want 1", got)
	}
}

func TestTickOnce_NoMBIDOnTrack_Skipped(t *testing.T) {
	f := newFixture(t)
	trackNoMbid := seedTrack(t, f, "NoMbid", nil)
	markPlayed(t, f, trackNoMbid.ID)
	// LB stub would return rows if queried; if worker queries it, the test
	// fails (any non-zero rows means we hit LB for a no-MBID track).
	srv := stubLB(`[{"recording_mbid":"11111111-1111-1111-1111-111111111111","score":0.9}]`, `[]`, http.StatusOK)
	defer srv.Close()
	w := newTestWorker(f, srv.URL)
	if err := w.tickOnce(context.Background()); err != nil {
		t.Fatalf("tickOnce: %v", err)
	}
	if got := countTrackSim(t, f, trackNoMbid.ID); got != 0 {
		t.Errorf("no-MBID track produced rows: %d", got)
	}
}

// leftPad pads an integer with leading zeros to width n, used to build
// deterministic MBID-like strings in tests.
func leftPad(v, width int) string {
	s := ""
	for i := 0; i < width; i++ {
		s = "0" + s
	}
	digits := ""
	for v > 0 {
		digits = string(rune('0'+v%10)) + digits
		v /= 10
	}
	if len(digits) >= width {
		return digits
	}
	return s[:width-len(digits)] + digits
}
```

- [ ] **Step 2: Run tests to verify they fail**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && MINSTREL_TEST_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' go test ./internal/similarity/ -v`

Expected: tests run but most fail because `tickOnce` is a stub returning nil.

- [ ] **Step 3: Implement tickOnce**

Replace the `tickOnce` stub in `internal/similarity/worker.go` with the full implementation. Add the imports `errors`, `git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq`, `github.com/jackc/pgx/v5/pgtype` to the import block:

```go
import (
	"context"
	"errors"
	"log/slog"
	"sort"
	"time"

	"github.com/jackc/pgx/v5/pgtype"
	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/scrobble/listenbrainz"
)
```

Replace `tickOnce` with:

```go
// tickOnce drains one batch of tracks and one batch of artists. Per-row
// errors are logged and skipped (passive retry via timer). 429 aborts the
// entire tick (the next hourly tick is far longer than any LB Retry-After).
//
// Returns the first error that aborts the whole tick (currently only 429);
// nil otherwise.
func (w *Worker) tickOnce(ctx context.Context) error {
	q := dbq.New(w.pool)
	if err := w.tickTracks(ctx, q); err != nil {
		return err
	}
	return w.tickArtists(ctx, q)
}

func (w *Worker) tickTracks(ctx context.Context, q *dbq.Queries) error {
	rows, err := q.ListPlayedTracksNeedingSimilarity(ctx, w.batch)
	if err != nil {
		return err
	}
	for _, r := range rows {
		if r.Mbid == nil {
			continue // defensive — query already filters NULL
		}
		results, err := w.client.SimilarRecordings(ctx, *r.Mbid, 100)
		if err != nil {
			var ra *listenbrainz.RetryAfterError
			if errors.As(err, &ra) {
				w.logger.Warn("similarity: 429 — aborting tick", "retry_after", ra.Wait)
				return nil // abort this tick; passive retry on next
			}
			w.logger.Warn("similarity: similar-recordings failed", "track_id", r.ID, "err", err)
			continue
		}
		w.upsertTrackSimilar(ctx, q, r.ID, results)
	}
	return nil
}

func (w *Worker) tickArtists(ctx context.Context, q *dbq.Queries) error {
	rows, err := q.ListPlayedArtistsNeedingSimilarity(ctx, w.batch)
	if err != nil {
		return err
	}
	for _, r := range rows {
		if r.Mbid == nil {
			continue
		}
		results, err := w.client.SimilarArtists(ctx, *r.Mbid, 100)
		if err != nil {
			var ra *listenbrainz.RetryAfterError
			if errors.As(err, &ra) {
				w.logger.Warn("similarity: 429 — aborting tick", "retry_after", ra.Wait)
				return nil
			}
			w.logger.Warn("similarity: similar-artists failed", "artist_id", r.ID, "err", err)
			continue
		}
		w.upsertArtistSimilar(ctx, q, r.ID, results)
	}
	return nil
}

// upsertTrackSimilar filters returned MBIDs to those in our library, takes
// top-K by score, and upserts rows.
func (w *Worker) upsertTrackSimilar(ctx context.Context, q *dbq.Queries, trackAID pgtype.UUID, results []listenbrainz.SimilarRecording) {
	if len(results) == 0 {
		return
	}
	// Sort by score desc (LB returns sorted; defensive).
	sort.Slice(results, func(i, j int) bool { return results[i].Score > results[j].Score })

	// Bulk-resolve MBIDs to local track IDs.
	mbids := make([]string, 0, len(results))
	for _, r := range results {
		mbids = append(mbids, r.MBID)
	}
	rows, err := q.GetTracksByMBIDs(ctx, mbids)
	if err != nil {
		w.logger.Warn("similarity: GetTracksByMBIDs", "err", err)
		return
	}
	idByMBID := make(map[string]pgtype.UUID, len(rows))
	for _, r := range rows {
		if r.Mbid != nil {
			idByMBID[*r.Mbid] = r.ID
		}
	}

	// Walk results in score order, take first K that map to a local track.
	taken := 0
	for _, r := range results {
		if taken >= w.topK {
			break
		}
		localID, ok := idByMBID[r.MBID]
		if !ok {
			continue
		}
		if localID == trackAID {
			continue // defensive — DB CHECK constraint also catches self-edges
		}
		if uerr := q.UpsertTrackSimilarity(ctx, dbq.UpsertTrackSimilarityParams{
			TrackAID: trackAID, TrackBID: localID, Score: r.Score,
		}); uerr != nil {
			w.logger.Warn("similarity: UpsertTrackSimilarity", "err", uerr)
			continue
		}
		taken++
	}
}

func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artistAID pgtype.UUID, results []listenbrainz.SimilarArtist) {
	if len(results) == 0 {
		return
	}
	sort.Slice(results, func(i, j int) bool { return results[i].Score > results[j].Score })

	mbids := make([]string, 0, len(results))
	for _, r := range results {
		mbids = append(mbids, r.MBID)
	}
	rows, err := q.GetArtistsByMBIDs(ctx, mbids)
	if err != nil {
		w.logger.Warn("similarity: GetArtistsByMBIDs", "err", err)
		return
	}
	idByMBID := make(map[string]pgtype.UUID, len(rows))
	for _, r := range rows {
		if r.Mbid != nil {
			idByMBID[*r.Mbid] = r.ID
		}
	}

	taken := 0
	for _, r := range results {
		if taken >= w.topK {
			break
		}
		localID, ok := idByMBID[r.MBID]
		if !ok {
			continue
		}
		if localID == artistAID {
			continue
		}
		if uerr := q.UpsertArtistSimilarity(ctx, dbq.UpsertArtistSimilarityParams{
			ArtistAID: artistAID, ArtistBID: localID, Score: r.Score,
		}); uerr != nil {
			w.logger.Warn("similarity: UpsertArtistSimilarity", "err", uerr)
			continue
		}
		taken++
	}
}
```

Note: the exact param-struct field names (`TrackAID`, `ArtistAID`, etc.) are emitted by sqlc — they may use different capitalization. After running `make generate`, inspect `internal/db/dbq/similarity.sql.go` and adjust the field references to match. Same for `r.Mbid` — sqlc may name the column type pointer field `Mbid` or `MBID`. Use whatever the generated code shows.

- [ ] **Step 4: Run tests to verify they pass**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && MINSTREL_TEST_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' go test ./internal/similarity/ -v`

Expected: PASS for all 9 integration tests + 1 constructor test.

- [ ] **Step 5: Verify lint clean**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./internal/similarity/...`

Expected: clean.

- [ ] **Step 6: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add internal/similarity/worker.go internal/similarity/worker_integration_test.go
git commit -m "feat(similarity): implement Worker.tickOnce with track + artist passes"
```

---

## Task 7: Boot the worker in main.go

**Files:**
- Modify: `cmd/minstrel/main.go`

- [ ] **Step 1: Add the import**

In `cmd/minstrel/main.go`, append to the existing import block:

```go
"git.fabledsword.com/bvandeusen/minstrel/internal/similarity"
```

(The `internal/scrobble/listenbrainz` import already exists from M4a; we re-use the same client.)

- [ ] **Step 2: Add the worker startup**

In `cmd/minstrel/main.go`, find where the M4a scrobble worker is started (a `scrobbleWorker := scrobble.NewWorker(...)` followed by `go scrobbleWorker.Run(ctx)`). Append immediately after:

```go
// Start the similarity ingest worker. Per spec §M4b, runs every 1h, drains
// up to 5 played tracks + 5 played artists per tick, weekly re-fetch cap
// per row. Public LB endpoints — no token required.
similarityWorker := similarity.NewWorker(pool, listenbrainz.NewClient(), logger.With("component", "similarity"))
go similarityWorker.Run(ctx)
```

- [ ] **Step 3: Verify compile**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go build ./...`

Expected: clean.

- [ ] **Step 4: Verify the worker actually starts (smoke test)**

Run a brief end-to-end check (re-uses the M4a smoke pattern):

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
SMARTMUSIC_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' \
  SMARTMUSIC_LIBRARY_SCAN_PATHS='' \
  SMARTMUSIC_LIBRARY_SCAN_ON_STARTUP=false \
  SMARTMUSIC_SERVER_ADDRESS=:14533 \
  timeout 5 go run ./cmd/minstrel/ 2>&1 | head -20
```

Expected: server starts, no panic. The similarity worker doesn't log per tick (only on errors), so the smoke is just "no crash."

- [ ] **Step 5: Verify lint clean**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./...`

Expected: clean.

- [ ] **Step 6: Commit**

```bash
cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel
git add cmd/minstrel/main.go
git commit -m "feat(cmd): start similarity worker alongside HTTP server"
```

---

## Task 8: Final verification + branch finish

**Files:** none (verification only)

- [ ] **Step 1: Full Go test suite (-short -race)**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && go test -short -race ./...`

Expected: PASS across all 16 packages (now including `internal/similarity`).

- [ ] **Step 2: Full integration suite**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && MINSTREL_TEST_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' go test -race -p 1 ./...`

Expected: PASS apart from the pre-existing `TestScanner_Integration` flake (CI runs `-short` which skips it; not introduced by this PR).

- [ ] **Step 3: Lint**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && golangci-lint run ./...`

Expected: clean.

- [ ] **Step 4: Coverage check on new package**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && MINSTREL_TEST_DATABASE_URL='postgres://minstrel:minstrel@172.22.0.2:5432/minstrel?sslmode=disable' go test -p 1 -coverprofile=/tmp/cover-m4b.out ./internal/similarity/... ./internal/scrobble/listenbrainz/... && go tool cover -func=/tmp/cover-m4b.out | tail -5`

Expected: `internal/similarity` ≥ 75%, `internal/scrobble/listenbrainz` ≥ 85%. The new `Similar*` methods are added on top of M4a's coverage.

- [ ] **Step 5: Web verification (no changes — sanity)**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel/web && npm run check && npm test -- --run && npm run build`

Expected: svelte-check 0/0; 180 vitest tests pass; build succeeds. (M4b doesn't touch web.)

- [ ] **Step 6: Docker smoke**

Run: `cd /home/bvandeusen/Nextcloud/Projects/Minstrel/minstrel && docker build -t minstrel:m4b-smoke .`

Expected: container builds.

- [ ] **Step 7: Manual verification post-merge (optional, deferred)**

Note: full validation requires a real LB-tagged library plus 1h+ uptime. The integration tests provide deterministic coverage. Manual verification at the user's discretion:

1. `docker compose up --build -d minstrel` — pick up new code
2. Wait ≥1 hour
3. `psql -c "SELECT count(*) FROM track_similarity WHERE source = 'listenbrainz'"` → non-zero rows IF library has MBID-tagged played tracks

- [ ] **Step 8: Update Fable task #346**

After PR opens, mark `in_progress`. After PR merges, mark `done` with a summary of what shipped.

- [ ] **Step 9: Finishing the branch**

**REQUIRED SUB-SKILL:** Use `superpowers:finishing-a-development-branch` to verify tests, present completion options, and execute the user's choice.

Per established cadence: this slice will land as a single-purpose PR. After merge, M4b closes; M4c (radio similarity-driven candidate pool + queue refresh, closes M4) is next.

---

## Self-Review

**Spec coverage:**
- §3.1 (new package `internal/similarity`) → Tasks 5, 6 ✓
- §3.2 (LB client extensions) → Tasks 3, 4; boot wiring → Task 7 ✓
- §3.3 (failure handling — passive retry, 429 abort tick) → Task 6 (worker logic + integration tests) ✓
- §4 (database schema) → Task 1 ✓
- §5 (sqlc queries) → Task 2 ✓
- §6 (worker algorithm) → Task 6 ✓
- §7.1 (LB client unit tests) → Tasks 3, 4 ✓
- §7.2 (worker integration tests) → Task 6 ✓
- §7.3 (coverage target) → Task 8 step 4 ✓
- §7.4 (manual verification) → Task 8 step 7 ✓
- §8 (backwards compat) → preserved by additive design (new tables, new package) ✓

No gaps.

**Placeholder scan:** No "TBD"/"TODO" content. The "use whatever sqlc emits at make generate time" guidance in Task 6 is a known sqlc-specific instruction, not a placeholder.

**Type consistency:**
- `Worker` struct fields (`pool`, `client`, `logger`, `tick`, `batch`, `topK`) — same in Task 5 (skeleton) and Task 6 (full impl).
- `tickOnce(ctx) error` — same signature throughout.
- `SimilarRecording`/`SimilarArtist` types defined in Tasks 3/4, consumed in Task 6.
- sqlc-generated method names referenced consistently: `ListPlayedTracksNeedingSimilarity`, `ListPlayedArtistsNeedingSimilarity`, `GetTracksByMBIDs`, `GetArtistsByMBIDs`, `UpsertTrackSimilarity`, `UpsertArtistSimilarity`.
- Constants `lbSimilarRecordingsAlgorithm` / `lbSimilarArtistsAlgorithm` defined in Task 3, both referenced from Tasks 3 and 4.