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/runner/runner.go

@@ -44,6 +44,9 @@ type Config struct {
 	// StaleTaskThreshold is how old a heartbeat must be (or absent) before the
 	// task is considered orphaned and reset to pending. Defaults to 2m when 0.
 	StaleTaskThreshold time.Duration
+	// BrowseRefreshInterval is how often the runner pre-fetches browse page
+	// snapshots from novelfire.net and stores them in MinIO. Defaults to 6h.
+	BrowseRefreshInterval time.Duration
 }
 
 // Dependencies are the external services the runner depends on.
@@ -56,6 +59,8 @@ type Dependencies struct {
 	BookReader bookstore.BookReader
 	// AudioStore persists generated audio and checks key existence.
 	AudioStore bookstore.AudioStore
+	// BrowseStore stores browse page snapshots in MinIO.
+	BrowseStore bookstore.BrowseStore
 	// Novel is the scraper implementation.
 	Novel scraper.NovelScraper
 	// Kokoro is the TTS client.
@@ -91,6 +96,9 @@ func New(cfg Config, deps Dependencies) *Runner {
 	if cfg.StaleTaskThreshold <= 0 {
 		cfg.StaleTaskThreshold = 2 * time.Minute
 	}
+	if cfg.BrowseRefreshInterval <= 0 {
+		cfg.BrowseRefreshInterval = 6 * time.Hour
+	}
 	if deps.Log == nil {
 		deps.Log = slog.Default()
 	}
@@ -121,6 +129,7 @@ func (r *Runner) Run(ctx context.Context) error {
 		"poll_interval", r.cfg.PollInterval,
 		"max_scrape", r.cfg.MaxConcurrentScrape,
 		"max_audio", r.cfg.MaxConcurrentAudio,
+		"browse_refresh_interval", r.cfg.BrowseRefreshInterval,
 	)
 
 	scrapeSem := make(chan struct{}, r.cfg.MaxConcurrentScrape)
@@ -134,6 +143,12 @@ func (r *Runner) Run(ctx context.Context) error {
 	tick := time.NewTicker(r.cfg.PollInterval)
 	defer tick.Stop()
 
+	browseTick := time.NewTicker(r.cfg.BrowseRefreshInterval)
+	defer browseTick.Stop()
+
+	// Run one browse refresh and one poll immediately on startup.
+	go r.runBrowseRefresh(ctx)
+
 	// Run one poll immediately on startup, then on each tick.
 	for {
 		r.poll(ctx, scrapeSem, audioSem, &wg)
@@ -154,6 +169,8 @@ func (r *Runner) Run(ctx context.Context) error {
 				r.deps.Log.Warn("runner: drain timeout exceeded, forcing exit")
 			}
 			return nil
+		case <-browseTick.C:
+			go r.runBrowseRefresh(ctx)
 		case <-tick.C:
 		}
 	}