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/bookstore/bookstore.go

@@ -99,6 +99,10 @@ type AudioStore interface {
 	// PutAudio stores raw audio bytes under the given MinIO object key.
 	PutAudio(ctx context.Context, key string, data []byte) error
 
+	// AudioObjectSize returns the byte size of the audio object at key,
+	// or -1 if the object does not exist or size cannot be determined.
+	AudioObjectSize(ctx context.Context, key string) int64
+
 	// 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" or "audio/wav".

backend/internal/bookstore/bookstore_test.go

@@ -57,6 +57,7 @@ func (m *mockStore) AudioObjectKey(_ string, _ int, _ string) string       { ret
 func (m *mockStore) AudioObjectKeyExt(_ 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) AudioObjectSize(_ context.Context, _ string) int64     { return -1 }
 func (m *mockStore) PutAudioStream(_ context.Context, _ string, _ io.Reader, _ int64, _ string) error {
 	return nil
 }