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>

.gitignore

@@ -29,3 +29,6 @@ Thumbs.db
 *.swo
 *~
 .opencode/
+
+# ── Playwright MCP browser session data ────────────────────────────────────────
+.playwright-mcp/