fix(storage): surface all silent errors with structured logging

- Inject *slog.Logger into HybridStore, PocketBaseStore, and pbClient
- Fix credential defaults in main.go (changeme123 / admin) to match docker-compose
- listOne/listAll/upsert/deleteWhere now return errors on non-2xx HTTP status
- WriteChapter: log warn instead of discarding UpsertChapterIdx error
- MetadataMtime, GetAudioCache, GetProgress: log warn on PocketBase failures
- EnsureCollections: log info/debug/warn per outcome instead of _ = err
- CountChapterIdx: log warn on failure instead of silently returning 0
- server: log warn when SetAudioCache fails after audio generation
- NewHybridStore: add explicit Ping() before EnsureCollections for fast-fail on bad credentials

scraper/internal/server/server.go

@@ -428,7 +428,9 @@ func (s *Server) handleAudioGenerate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	_ = s.store.SetAudioCache(r.Context(), cacheKey, filename)
+	if err := s.store.SetAudioCache(r.Context(), cacheKey, filename); err != nil {
+		s.log.Warn("audio cache write failed", "slug", slug, "chapter", n, "cache_key", cacheKey, "err", err)
+	}
 
 	// Download generated audio from Kokoro and persist to MinIO so that
 	// presigned URLs for the audio object are accessible.