Add async audio generation: job tracking in PocketBase + UI polling

Replace blocking POST /api/audio with a non-blocking 202 flow: the Go
handler immediately enqueues a job in a new `audio_jobs` PocketBase
collection and returns {job_id, status}. A background goroutine runs
the actual Kokoro TTS work and updates job status (pending → generating
→ done/failed). A new GET /api/audio/status/{slug}/{n} endpoint lets
clients poll progress. The SvelteKit proxy and AudioPlayer.svelte are
updated to POST, then poll the status route every 2s until done.

scraper/internal/server/server.go

@@ -11,7 +11,8 @@
 //	GET  /api/presign/chapter/{slug}/{n}  — presigned MinIO URL for chapter markdown
 //	GET  /api/presign/audio/{slug}/{n}    — presigned MinIO URL for chapter audio
 //	GET  /api/chapter-text/{slug}/{n}     — plain text of chapter (markdown stripped)
-//	POST /api/audio/{slug}/{n}            — trigger Kokoro audio generation
+//	POST /api/audio/{slug}/{n}            — trigger Kokoro audio generation (async, returns 202)
+//	GET  /api/audio/status/{slug}/{n}     — poll audio generation job status
 //	GET  /api/audio-proxy/{slug}/{n}      — proxy generated audio from Kokoro
 package server
 
@@ -47,11 +48,11 @@ type Server struct {
 	voiceMu      sync.RWMutex
 	cachedVoices []string // populated on first request from Kokoro /v1/audio/voices
 
-	// audioMu guards audioInFlight only.
+	// audioMu guards audioJobIDs only.
 	// Completed audio filenames are persisted to the Store (PocketBase).
-	// audioInFlight deduplicates concurrent generation requests for the same key.
-	audioMu       sync.Mutex
-	audioInFlight map[string]chan struct{} // cacheKey → closed when done
+	// audioJobIDs deduplicates concurrent generation requests for the same key.
+	audioMu     sync.Mutex
+	audioJobIDs map[string]string // cacheKey → PocketBase job ID (empty string if record creation failed)
 
 	// browseMu guards browseInFlight — keys currently being refreshed
 	// in the background.
@@ -82,7 +83,7 @@ func New(addr string, oCfg orchestrator.Config, novel scraper.NovelScraper, log
 		store:          store,
 		kokoroURL:      kokoroURL,
 		kokoroVoice:    kokoroVoice,
-		audioInFlight:  make(map[string]chan struct{}),
+		audioJobIDs:    make(map[string]string),
 		browseInFlight: make(map[string]struct{}),
 		browseMemCache: make(map[string]browseCacheEntry),
 	}
@@ -177,13 +178,11 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	)
 	mux.Handle("POST /api/audio/voice-samples", voiceSampleHandler)
 	// Server-side audio generation via Kokoro /v1/audio/speech.
-	// Generation can take several minutes, so wrap in its own timeout handler.
-	audioGenHandler := http.TimeoutHandler(
-		http.HandlerFunc(s.handleAudioGenerate),
-		10*time.Minute,
-		`{"error":"audio generation timed out"}`,
-	)
-	mux.Handle("POST /api/audio/{slug}/{n}", audioGenHandler)
+	// POST returns 202 immediately and starts a background goroutine;
+	// poll GET /api/audio/status/{slug}/{n} to track progress.
+	mux.HandleFunc("POST /api/audio/{slug}/{n}", s.handleAudioGenerate)
+	// Audio job status polling endpoint.
+	mux.HandleFunc("GET /api/audio/status/{slug}/{n}", s.handleAudioStatus)
 	// Proxy route: fetches the generated file from Kokoro /v1/download/{filename}.
 	mux.HandleFunc("GET /api/audio-proxy/{slug}/{n}", s.handleAudioProxy)