fix: chunk large chapter text for Kokoro TTS to prevent EOF on big inputs

Split chapter text into ~1000-char sentence-boundary chunks before sending
to kokoro-fastapi. Each chunk is generated individually and the raw MP3 bytes
are concatenated. This prevents the EOF / timeout failures that occur when
the server receives a very large single request (e.g. a full PDF 'Full Text'
chapter). chunkText() breaks at sentence endings (. ! ? newlines) to preserve
natural speech flow.

backend/internal/runner/runner.go

@@ -656,7 +656,7 @@ func (r *Runner) runAudioTask(ctx context.Context, task domain.AudioTask) {
 			return
 		}
 		var genErr error
-		audioData, genErr = r.deps.Kokoro.GenerateAudio(ctx, text, task.Voice)
+		audioData, genErr = kokoroGenerateChunked(ctx, r.deps.Kokoro, text, task.Voice, log)
 		if genErr != nil {
 			fail(fmt.Sprintf("kokoro generate: %v", genErr))
 			return
@@ -685,6 +685,31 @@ func (r *Runner) runAudioTask(ctx context.Context, task domain.AudioTask) {
 	log.Info("runner: audio task finished", "key", key)
 }
 
+// kokoroGenerateChunked splits text into ~1 000-character sentence-boundary
+// chunks, calls Kokoro.GenerateAudio for each, and concatenates the raw MP3
+// bytes. This avoids EOF / timeout failures that occur when the Kokoro
+// FastAPI server receives very large inputs (e.g. a full imported PDF chapter).
+//
+// Concatenating raw MP3 frames is valid — MP3 is a frame-based format and
+// standard players handle multi-segment files correctly.
+func kokoroGenerateChunked(ctx context.Context, k kokoro.Client, text, voice string, log *slog.Logger) ([]byte, error) {
+	const chunkSize = 1000
+
+	chunks := chunkText(text, chunkSize)
+	log.Info("runner: kokoro chunked generation", "chunks", len(chunks), "total_chars", len(text))
+
+	var combined []byte
+	for i, chunk := range chunks {
+		data, err := k.GenerateAudio(ctx, chunk, voice)
+		if err != nil {
+			return nil, fmt.Errorf("chunk %d/%d: %w", i+1, len(chunks), err)
+		}
+		combined = append(combined, data...)
+		log.Info("runner: kokoro chunk done", "chunk", i+1, "of", len(chunks), "bytes", len(data))
+	}
+	return combined, nil
+}
+
 // runImportTask executes one PDF/EPUB import task.
 // Preferred path: when task.ChaptersKey is set, it reads pre-parsed chapters
 // JSON from MinIO (written by the backend at upload time) and ingests them.