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/bookstore/bookstore.go

@@ -123,3 +123,15 @@ type ProgressStore interface {
 	// DeleteProgress removes progress for a specific slug.
 	DeleteProgress(ctx context.Context, sessionID, slug string) error
 }
+
+// BrowseStore covers browse page snapshot storage.
+// The runner writes snapshots; the backend reads them.
+type BrowseStore interface {
+	// PutBrowsePage stores a raw JSON snapshot for a browse page.
+	// genre, sort, status, novelType and page identify the page.
+	PutBrowsePage(ctx context.Context, genre, sort, status, novelType string, page int, data []byte) error
+
+	// GetBrowsePage retrieves a raw JSON snapshot. Returns (nil, false, nil)
+	// when no snapshot exists for the given parameters.
+	GetBrowsePage(ctx context.Context, genre, sort, status, novelType string, page int) ([]byte, bool, error)
+}