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_test.go

@@ -210,3 +210,29 @@ func TestQuarantineReasonLabel(t *testing.T) {
 		}
 	}
 }
+
+func TestNotify_PlaybackErrorsCoalesceIntoOneCount(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+
+	alice := seedUser(t, pool, "np-play-alice", "pw", false)
+	admin := seedUser(t, pool, "np-play-admin", "pw", true)
+	track := seedQuarantineTrack(t, h, "play")
+
+	for i := 0; i < 2; i++ {
+		body := fmt.Sprintf(`{"track_id":%q,"kind":"load_failed","client_id":"web-%d"}`, uuidToString(track.ID), i)
+		req := withUser(httptest.NewRequest(http.MethodPost, "/api/playback-errors", strings.NewReader(body)), alice)
+		w := httptest.NewRecorder()
+		h.handleReportPlaybackError(w, req)
+		if w.Code != http.StatusCreated {
+			t.Fatalf("report %d: status = %d; body = %s", i, w.Code, w.Body.String())
+		}
+	}
+
+	got := inboxOf(t, pool, admin)
+	wantInbox(t, "admin", got, notifications.KindPlaybackErrors)
+	if got[0].Payload.Count != 2 {
+		t.Errorf("count = %d, want 2 unresolved", got[0].Payload.Count)
+	}
+	wantInbox(t, "reporter", inboxOf(t, pool, alice))
+}