feat: streaming audio endpoint with MinIO write-through cache

Add GET /api/audio-stream/{slug}/{n}?voice= that streams MP3 audio to the
client as TTS generates it, while simultaneously uploading to MinIO. On
subsequent requests the endpoint redirects to the presigned MinIO URL,
skipping generation entirely.

- PocketTTS: StreamAudioMP3 pipes live WAV response body through ffmpeg
  (streaming transcode — no full-buffer wait)
- Kokoro: StreamAudioMP3 uses stream:true mode, returning MP3 frames
  directly without the two-step download-link flow
- AudioStore: PutAudioStream added for multipart MinIO upload from reader
- WriteTimeout bumped 60s → 15min to accommodate full-chapter streams
- X-Accel-Buffering: no header disables Caddy/nginx response buffering

backend/internal/bookstore/bookstore.go

@@ -14,6 +14,7 @@ package bookstore
 
 import (
 	"context"
+	"io"
 	"time"
 
 	"github.com/libnovel/backend/internal/domain"
@@ -87,6 +88,11 @@ type AudioStore interface {
 
 	// PutAudio stores raw audio bytes under the given MinIO object key.
 	PutAudio(ctx context.Context, key string, data []byte) error
+
+	// PutAudioStream uploads audio from r to MinIO under key.
+	// size must be the exact byte length of r, or -1 to use multipart upload.
+	// contentType should be "audio/mpeg".
+	PutAudioStream(ctx context.Context, key string, r io.Reader, size int64, contentType string) error
 }
 
 // PresignStore generates short-lived URLs — used exclusively by the backend.

backend/internal/bookstore/bookstore_test.go

@@ -2,6 +2,7 @@ package bookstore_test
 
 import (
 	"context"
+	"io"
 	"testing"
 	"time"
 
@@ -54,6 +55,9 @@ func (m *mockStore) RankingFreshEnough(_ context.Context, _ time.Duration) (bool
 func (m *mockStore) AudioObjectKey(_ string, _ int, _ string) string      { return "" }
 func (m *mockStore) AudioExists(_ context.Context, _ string) bool         { return false }
 func (m *mockStore) PutAudio(_ context.Context, _ string, _ []byte) error { return nil }
+func (m *mockStore) PutAudioStream(_ context.Context, _ string, _ io.Reader, _ int64, _ string) error {
+	return nil
+}
 
 // PresignStore
 func (m *mockStore) PresignChapter(_ context.Context, _ string, _ int, _ time.Duration) (string, error) {