fix: serve browse pages from MinIO cache; fix ReapStaleTasks PocketBase filter

- Runner fetches 9 browse combos (genre×sort×status) every 6h and stores
  JSON snapshots in MinIO libnovel-browse bucket (browse_refresh.go)
- Backend handleBrowse reads page-1 results from MinIO first; falls back
  to live novelfire.net fetch; returns empty+cached:false on total failure
  instead of 502
- Add BrowseStore interface (bookstore.go), MinIO put/get helpers (minio.go),
  Store methods + compile-time assertion (store.go), BucketBrowse config,
  wiring in cmd/backend and cmd/runner, docker-compose-new bucket init
- Fix ReapStaleTasks: PocketBase datetime fields require heartbeat_at=null
  (not heartbeat_at="") in filter expressions, and nil (not "") in patch
  payload — was causing 400 errors on every reap cycle

backend/internal/storage/minio.go

@@ -22,6 +22,7 @@ type minioClient struct {
 	bucketChapters string
 	bucketAudio    string
 	bucketAvatars  string
+	bucketBrowse   string
 }
 
 func newMinioClient(cfg config.MinIO) (*minioClient, error) {
@@ -78,12 +79,13 @@ func newMinioClient(cfg config.MinIO) (*minioClient, error) {
 		bucketChapters: cfg.BucketChapters,
 		bucketAudio:    cfg.BucketAudio,
 		bucketAvatars:  cfg.BucketAvatars,
+		bucketBrowse:   cfg.BucketBrowse,
 	}, nil
 }
 
 // ensureBuckets creates all required buckets if they don't already exist.
 func (m *minioClient) ensureBuckets(ctx context.Context) error {
-	for _, bucket := range []string{m.bucketChapters, m.bucketAudio, m.bucketAvatars} {
+	for _, bucket := range []string{m.bucketChapters, m.bucketAudio, m.bucketAvatars, m.bucketBrowse} {
 		exists, err := m.client.BucketExists(ctx, bucket)
 		if err != nil {
 			return fmt.Errorf("minio: check bucket %q: %w", bucket, err)
@@ -117,6 +119,12 @@ func AvatarObjectKey(userID, ext string) string {
 	return fmt.Sprintf("%s/%s.%s", userID, ext, ext)
 }
 
+// BrowseObjectKey returns the MinIO object key for a cached browse page snapshot.
+// Format: browse/{genre}/{sort}/{status}/{type}/page-{n}.json
+func BrowseObjectKey(genre, sort, status, novelType string, page int) string {
+	return fmt.Sprintf("browse/%s/%s/%s/%s/page-%d.json", genre, sort, status, novelType, page)
+}
+
 // chapterNumberFromKey extracts the chapter number from a MinIO object key.
 // e.g. "my-book/chapter-000042.md" → 42
 func chapterNumberFromKey(key string) int {
@@ -192,3 +200,23 @@ func (m *minioClient) listObjectKeys(ctx context.Context, bucket, prefix string)
 	}
 	return keys, nil
 }
+
+// ── Browse operations ─────────────────────────────────────────────────────────
+
+// putBrowse stores raw JSON bytes for a browse page snapshot.
+func (m *minioClient) putBrowse(ctx context.Context, key string, data []byte) error {
+	return m.putObject(ctx, m.bucketBrowse, key, "application/json", data)
+}
+
+// getBrowse retrieves a browse page snapshot. Returns (nil, false, nil) when
+// the object does not exist.
+func (m *minioClient) getBrowse(ctx context.Context, key string) ([]byte, bool, error) {
+	if !m.objectExists(ctx, m.bucketBrowse, key) {
+		return nil, false, nil
+	}
+	data, err := m.getObject(ctx, m.bucketBrowse, key)
+	if err != nil {
+		return nil, false, err
+	}
+	return data, true, nil
+}

backend/internal/storage/store.go

@@ -50,6 +50,7 @@ var _ bookstore.RankingStore = (*Store)(nil)
 var _ bookstore.AudioStore = (*Store)(nil)
 var _ bookstore.PresignStore = (*Store)(nil)
 var _ bookstore.ProgressStore = (*Store)(nil)
+var _ bookstore.BrowseStore = (*Store)(nil)
 var _ taskqueue.Producer = (*Store)(nil)
 var _ taskqueue.Consumer = (*Store)(nil)
 var _ taskqueue.Reader = (*Store)(nil)
@@ -608,12 +609,13 @@ func (s *Store) HeartbeatTask(ctx context.Context, id string) error {
 // re-claimed. Returns the number of tasks reaped.
 func (s *Store) ReapStaleTasks(ctx context.Context, staleAfter time.Duration) (int, error) {
 	threshold := time.Now().UTC().Add(-staleAfter).Format(time.RFC3339)
-	// Match tasks that are running AND (heartbeat_at is empty OR heartbeat_at < threshold).
-	filter := fmt.Sprintf(`status="running"&&(heartbeat_at=""||heartbeat_at<"%s")`, threshold)
+	// Match tasks that are running AND (heartbeat_at is null OR heartbeat_at < threshold).
+	// PocketBase datetime fields require `=null` not `=""` in filter expressions.
+	filter := fmt.Sprintf(`status="running"&&(heartbeat_at=null||heartbeat_at<"%s")`, threshold)
 	resetPayload := map[string]any{
 		"status":       string(domain.TaskStatusPending),
 		"worker_id":    "",
-		"heartbeat_at": "",
+		"heartbeat_at": nil,
 	}
 
 	total := 0
@@ -767,3 +769,22 @@ func parseAudioTask(raw json.RawMessage) (domain.AudioTask, error) {
 		Finished:     finished,
 	}, nil
 }
+
+// ── BrowseStore ────────────────────────────────────────────────────────────────
+
+func (s *Store) PutBrowsePage(ctx context.Context, genre, sort, status, novelType string, page int, data []byte) error {
+	key := BrowseObjectKey(genre, sort, status, novelType, page)
+	if err := s.mc.putBrowse(ctx, key, data); err != nil {
+		return fmt.Errorf("PutBrowsePage: %w", err)
+	}
+	return nil
+}
+
+func (s *Store) GetBrowsePage(ctx context.Context, genre, sort, status, novelType string, page int) ([]byte, bool, error) {
+	key := BrowseObjectKey(genre, sort, status, novelType, page)
+	data, ok, err := s.mc.getBrowse(ctx, key)
+	if err != nil {
+		return nil, false, fmt.Errorf("GetBrowsePage: %w", err)
+	}
+	return data, ok, nil
+}