feat(server): "You might like" album/artist Home rows (#790)

Surface in-library albums/artists the listener doesn't actively spin but
is predicted to enjoy, derived from the same similarity + like-weighted
candidate engine that powers For-You — rolled up from track scores to
album/artist granularity. Built in the daily 3am BuildSystemPlaylists
pass, atomic-replaced alongside the system playlists, and read back by
/api/home (+ /api/home/index).

Cold-start gate: skips generation entirely below 20 distinct unskipped
tracks AND 5 distinct artists, so a thin profile ships empty rows rather
than near-random tiles.

- migration 0034: you_might_like_albums / you_might_like_artists (id+rank,
  CASCADE, per-user rank index).
- playlists/you_might_like.go: cold-start gate + similarity roll-up
  (sum-of-top-3 aggregation, per-artist album cap, daily-rotating via the
  same userIDHash jitter as For-You) + atomic-replace persist in the tx.
- recommendation/home.go: two new HomePayload sections with read-time
  cross-section dedup vs Most Played / Rediscover / Last Played, trimmed
  to 10 each.
- api: you_might_like_albums / you_might_like_artists on /api/home and
  /api/home/index, reusing albumRefFrom / artistRefFromCovered.
- tests: pure roll-up/aggregation/cap unit tests + DB-backed gate,
  sufficiency, and atomic-replace tests (all green vs real Postgres).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/playlists/you_might_like_db_test.go

@@ -0,0 +1,99 @@
+package playlists_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
+)
+
+// countYouMightLike returns the persisted row counts for a user.
+func countYouMightLike(t *testing.T, pool *pgxpool.Pool, userID pgtype.UUID) (albums, artists int) {
+	t.Helper()
+	ctx := context.Background()
+	if err := pool.QueryRow(ctx,
+		`SELECT count(*) FROM you_might_like_albums WHERE user_id = $1`, userID,
+	).Scan(&albums); err != nil {
+		t.Fatalf("count albums: %v", err)
+	}
+	if err := pool.QueryRow(ctx,
+		`SELECT count(*) FROM you_might_like_artists WHERE user_id = $1`, userID,
+	).Scan(&artists); err != nil {
+		t.Fatalf("count artists: %v", err)
+	}
+	return albums, artists
+}
+
+// TestYouMightLike_ColdStartGate: a thin profile (3 artists × 3 tracks =
+// 9 distinct tracks, below the 20-track / 5-artist gate) must produce no
+// You-might-like rows — the build ships nothing rather than near-random
+// tiles.
+func TestYouMightLike_ColdStartGate(t *testing.T) {
+	pool := newPool(t)
+	logger := discardLogger()
+	u, _ := seedActiveLibrary(t, pool, "ymlcold", 3, 3)
+
+	if err := playlists.BuildSystemPlaylists(
+		context.Background(), pool, logger, u.ID, time.Now().UTC(), t.TempDir(),
+	); err != nil {
+		t.Fatalf("build: %v", err)
+	}
+
+	albums, artists := countYouMightLike(t, pool, u.ID)
+	if albums != 0 || artists != 0 {
+		t.Errorf("cold-start user should have no you-might-like rows; got %d albums, %d artists",
+			albums, artists)
+	}
+}
+
+// TestYouMightLike_SufficientActivityPopulates: a rich profile (8 artists
+// × 4 tracks = 32 distinct tracks, clearing both gate bars) populates the
+// You-might-like tables in the daily build.
+func TestYouMightLike_SufficientActivityPopulates(t *testing.T) {
+	pool := newPool(t)
+	logger := discardLogger()
+	u, _ := seedActiveLibrary(t, pool, "ymlrich", 8, 4)
+
+	if err := playlists.BuildSystemPlaylists(
+		context.Background(), pool, logger, u.ID, time.Now().UTC(), t.TempDir(),
+	); err != nil {
+		t.Fatalf("build: %v", err)
+	}
+
+	albums, artists := countYouMightLike(t, pool, u.ID)
+	if albums == 0 {
+		t.Error("rich user should have you-might-like album rows; got 0")
+	}
+	if artists == 0 {
+		t.Error("rich user should have you-might-like artist rows; got 0")
+	}
+}
+
+// TestYouMightLike_AtomicReplace: a second build replaces rather than
+// appends, so counts stay bounded by the persisted depth.
+func TestYouMightLike_AtomicReplace(t *testing.T) {
+	pool := newPool(t)
+	logger := discardLogger()
+	u, _ := seedActiveLibrary(t, pool, "ymlreplace", 8, 4)
+	ctx := context.Background()
+
+	for i := 0; i < 2; i++ {
+		if err := playlists.BuildSystemPlaylists(
+			ctx, pool, logger, u.ID, time.Now().UTC(), t.TempDir(),
+		); err != nil {
+			t.Fatalf("build %d: %v", i, err)
+		}
+	}
+
+	albums, artists := countYouMightLike(t, pool, u.ID)
+	if albums > 12 {
+		t.Errorf("albums = %d after two builds, want <= 12 (atomic replace)", albums)
+	}
+	if artists > 12 {
+		t.Errorf("artists = %d after two builds, want <= 12 (atomic replace)", artists)
+	}
+}