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/playback_errors.go

@@ -130,6 +130,7 @@ func (h *handlers) handleReportPlaybackError(w http.ResponseWriter, r *http.Requ
 		writeErr(w, apierror.Internal(err))
 		return
 	}
+	h.notifyPlaybackErrors(r.Context())
 	writeJSON(w, http.StatusCreated, map[string]string{"id": uuidToString(row.ID)})
 }