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/config/config.go

@@ -44,6 +44,8 @@ type MinIO struct {
 	BucketAudio string
 	// BucketAvatars is the bucket that holds user avatar images.
 	BucketAvatars string
+	// BucketBrowse is the bucket that holds cached browse page snapshots (JSON).
+	BucketBrowse string
 }
 
 // Kokoro holds connection settings for the Kokoro-FastAPI TTS service.
@@ -118,6 +120,7 @@ func Load() Config {
 			BucketChapters: envOr("MINIO_BUCKET_CHAPTERS", "libnovel-chapters"),
 			BucketAudio:    envOr("MINIO_BUCKET_AUDIO", "libnovel-audio"),
 			BucketAvatars:  envOr("MINIO_BUCKET_AVATARS", "libnovel-avatars"),
+			BucketBrowse:   envOr("MINIO_BUCKET_BROWSE", "libnovel-browse"),
 		},
 
 		Kokoro: Kokoro{