feat(similarity): add Worker skeleton with constructor + Run loop

internal/similarity/worker.go

@@ -0,0 +1,66 @@
+// 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(_ context.Context) error {
+	return nil
+}

internal/similarity/worker_test.go

@@ -0,0 +1,22 @@
+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)
+	}
+}