feat(notifications): library health reaches admins, coalesced (#5341)

- A failed scan run sends scan_failed. Each failure adds to the count and
  the notice shows the latest error. A scan cut short by shutdown says
  nothing.
- Marking tracks missing sends tracks_missing with a running count.
- A duplicate sweep that proposes a group it had not proposed before
  sends duplicates_found, counting everything awaiting review. A sweep
  that only re-finds known groups stays quiet, so a read notice isn't
  repeated every sweep (CountDuplicateGroupsDetectedSince).
- A playback-error report sends playback_errors, counting the unresolved
  errors (CountUnresolvedPlaybackErrors).

The library package gets its notifier as a package-level SetNotifier
beside SetEventBus, for the same reason the bus is package-level.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/notify_producers.go

@@ -71,3 +71,16 @@ func (h *handlers) trackLabel(ctx context.Context, trackID pgtype.UUID) string {
 	}
 	return t.Title
 }
+
+// notifyPlaybackErrors tells admins how many playback errors await review.
+// The notice coalesces, so while it is unread a new report only updates the
+// count.
+func (h *handlers) notifyPlaybackErrors(ctx context.Context) {
+	n, err := dbq.New(h.pool).CountUnresolvedPlaybackErrors(ctx)
+	if err != nil {
+		h.logger.Warn("api: playback_error: count unresolved", "err", err)
+		return
+	}
+	h.notifier.NotifyLogged(ctx, notifications.KindPlaybackErrors, notifications.ToAdmins(pgtype.UUID{}),
+		notifications.Payload{Count: n}.Map())
+}