feat(podcast): add podcast RSS feed generation and publishing

Admins can now generate TTS audiobooks chapter-by-chapter and publish
them as standard RSS 2.0 + iTunes podcast feeds that Spotify, Apple
Podcasts, and any podcast app can subscribe to.

Backend:
- POST /api/admin/podcast — creates ai_job (kind=podcast), spawns
  goroutine that generates TTS for missing chapters and writes to MinIO
- GET /podcast/{slug}.xml?voice=<id> — public RSS feed with correct
  pubDate (from chapters_idx.created) and enclosure length (MinIO stat)
- GET /podcast/audio/{slug}/{n}/{voice} — public audio proxy, 302 to
  presigned MinIO URL (no auth required for podcast clients)
- GET /api/admin/podcast/{slug} — list podcast jobs for a book
- Add AudioObjectSize to AudioStore interface backed by MinIO StatObject
- Populate ChapterInfo.Date from chapters_idx.created in ListChapters

UI:
- New /admin/podcast page: book + voice selector, chapter range, live
  progress bars, copy-feed-URL button, cancel button, how-to instructions
- /api/admin/podcast SvelteKit proxy (injects admin Bearer token)
- Podcast link added to admin sidebar

Cleanup:
- Delete stray playwright screenshot files from repo root
- Add .playwright-mcp/ to .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/storage/minio.go

@@ -190,6 +190,14 @@ func (m *minioClient) objectExists(ctx context.Context, bucket, key string) bool
 	return err == nil
 }
 
+func (m *minioClient) objectSize(ctx context.Context, bucket, key string) int64 {
+	info, err := m.client.StatObject(ctx, bucket, key, minio.StatObjectOptions{})
+	if err != nil {
+		return -1
+	}
+	return info.Size
+}
+
 func (m *minioClient) presignGet(ctx context.Context, bucket, key string, expires time.Duration) (string, error) {
 	u, err := m.pubClient.PresignedGetObject(ctx, bucket, key, expires, nil)
 	if err != nil {

backend/internal/storage/store.go

@@ -370,11 +370,16 @@ func (s *Store) ListChapters(ctx context.Context, slug string) ([]domain.Chapter
 	chapters := make([]domain.ChapterInfo, 0, len(items))
 	for _, raw := range items {
 		var rec struct {
-			Number int    `json:"number"`
-			Title  string `json:"title"`
+			Number  int    `json:"number"`
+			Title   string `json:"title"`
+			Created string `json:"created"`
 		}
 		json.Unmarshal(raw, &rec)
-		chapters = append(chapters, domain.ChapterInfo{Number: rec.Number, Title: rec.Title})
+		chapters = append(chapters, domain.ChapterInfo{
+			Number: rec.Number,
+			Title:  rec.Title,
+			Date:   rec.Created,
+		})
 	}
 	return chapters, nil
 }
@@ -606,6 +611,10 @@ func (s *Store) AudioExists(ctx context.Context, key string) bool {
 	return s.mc.objectExists(ctx, s.mc.bucketAudio, key)
 }
 
+func (s *Store) AudioObjectSize(ctx context.Context, key string) int64 {
+	return s.mc.objectSize(ctx, s.mc.bucketAudio, key)
+}
+
 func (s *Store) PutAudio(ctx context.Context, key string, data []byte) error {
 	return s.mc.putObject(ctx, s.mc.bucketAudio, key, "audio/mpeg", data)
 }