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

docker-compose-new.yml

@@ -30,6 +30,7 @@ services:
         mc mb --ignore-existing local/libnovel-chapters;
         mc mb --ignore-existing local/libnovel-audio;
         mc mb --ignore-existing local/libnovel-avatars;
+        mc mb --ignore-existing local/libnovel-browse;
         echo 'buckets ready';
       "
     environment:
@@ -88,7 +89,7 @@ services:
     environment:
       BACKEND_HTTP_ADDR: ":8080"
       LOG_LEVEL: "${LOG_LEVEL:-info}"
-      # MinIO
+       # MinIO
       MINIO_ENDPOINT: "minio:9000"
       MINIO_ACCESS_KEY: "${MINIO_ROOT_USER:-admin}"
       MINIO_SECRET_KEY: "${MINIO_ROOT_PASSWORD:-changeme123}"
@@ -96,6 +97,7 @@ services:
       MINIO_BUCKET_CHAPTERS: "${MINIO_BUCKET_CHAPTERS:-libnovel-chapters}"
       MINIO_BUCKET_AUDIO: "${MINIO_BUCKET_AUDIO:-libnovel-audio}"
       MINIO_BUCKET_AVATARS: "${MINIO_BUCKET_AVATARS:-libnovel-avatars}"
+      MINIO_BUCKET_BROWSE: "${MINIO_BUCKET_BROWSE:-libnovel-browse}"
       MINIO_PUBLIC_ENDPOINT: "${MINIO_PUBLIC_ENDPOINT:-http://localhost:9000}"
       MINIO_PUBLIC_USE_SSL: "${MINIO_PUBLIC_USE_SSL:-false}"
       # PocketBase
@@ -152,6 +154,7 @@ services:
       MINIO_BUCKET_CHAPTERS: "${MINIO_BUCKET_CHAPTERS:-libnovel-chapters}"
       MINIO_BUCKET_AUDIO: "${MINIO_BUCKET_AUDIO:-libnovel-audio}"
       MINIO_BUCKET_AVATARS: "${MINIO_BUCKET_AVATARS:-libnovel-avatars}"
+      MINIO_BUCKET_BROWSE: "${MINIO_BUCKET_BROWSE:-libnovel-browse}"
       MINIO_PUBLIC_ENDPOINT: "${MINIO_PUBLIC_ENDPOINT:-http://localhost:9000}"
       MINIO_PUBLIC_USE_SSL: "${MINIO_PUBLIC_USE_SSL:-false}"
       # PocketBase