feat(scrobble): add MaybeEnqueue with idempotent insert + threshold gate

Bridges closed play_events to the scrobble_queue: loads the event,
applies the Qualifies() threshold, checks user LB config (enabled + token),
and inserts via ON CONFLICT DO NOTHING for idempotency. Best-effort —
callers (playevents.Writer hooks, Task 8) will log and swallow errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/scrobble/queue.go

@@ -0,0 +1,49 @@
+package scrobble
+
+import (
+	"context"
+	"errors"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// MaybeEnqueue inserts a scrobble_queue row for the given play_event if:
+//  1. The user has listenbrainz_enabled = true with a non-empty token.
+//  2. The play_event passes Qualifies().
+//
+// Idempotent via UNIQUE(play_event_id) — re-runs are no-ops.
+//
+// Best-effort: callers (the playevents.Writer hooks) should log returned
+// errors but not propagate them, since scrobble delivery is enrichment
+// and must not block the canonical play history.
+func MaybeEnqueue(ctx context.Context, q *dbq.Queries, playEventID pgtype.UUID) error {
+	pe, err := q.GetPlayEventByID(ctx, playEventID)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil
+		}
+		return err
+	}
+	// Only closed events qualify (DurationPlayedMs/CompletionRatio populated).
+	if pe.DurationPlayedMs == nil || pe.CompletionRatio == nil {
+		return nil
+	}
+	if !Qualifies(int(*pe.DurationPlayedMs), *pe.CompletionRatio) {
+		return nil
+	}
+	cfg, err := q.GetListenBrainzConfig(ctx, pe.UserID)
+	if err != nil {
+		return err
+	}
+	if !cfg.ListenbrainzEnabled || cfg.ListenbrainzToken == nil || *cfg.ListenbrainzToken == "" {
+		return nil
+	}
+	_, err = q.EnqueueScrobble(ctx, dbq.EnqueueScrobbleParams{
+		UserID:      pe.UserID,
+		PlayEventID: pe.ID,
+	})
+	return err
+}