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/storage/minio.go

@@ -155,6 +155,14 @@ func (m *minioClient) putObject(ctx context.Context, bucket, key, contentType st
 	return err
 }
 
+// putObjectStream uploads from r with known size (or -1 for multipart).
+func (m *minioClient) putObjectStream(ctx context.Context, bucket, key, contentType string, r io.Reader, size int64) error {
+	_, err := m.client.PutObject(ctx, bucket, key, r, size,
+		minio.PutObjectOptions{ContentType: contentType},
+	)
+	return err
+}
+
 func (m *minioClient) getObject(ctx context.Context, bucket, key string) ([]byte, error) {
 	obj, err := m.client.GetObject(ctx, bucket, key, minio.GetObjectOptions{})
 	if err != nil {

backend/internal/storage/store.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"log/slog"
 	"strings"
 	"time"
@@ -383,6 +384,10 @@ func (s *Store) PutAudio(ctx context.Context, key string, data []byte) error {
 	return s.mc.putObject(ctx, s.mc.bucketAudio, key, "audio/mpeg", data)
 }
 
+func (s *Store) PutAudioStream(ctx context.Context, key string, r io.Reader, size int64, contentType string) error {
+	return s.mc.putObjectStream(ctx, s.mc.bucketAudio, key, contentType, r, size)
+}
+
 // ── PresignStore ──────────────────────────────────────────────────────────────
 
 func (s *Store) PresignChapter(ctx context.Context, slug string, n int, expires time.Duration) (string, error) {