Add BunnyCDN integration, on-air website badge, publish script fixes

- On-air toggle uploads status.json to BunnyCDN + purges cache, website
  polls it every 15s to show live ON AIR / OFF AIR badge
- Publish script downloads Castopod's copy of audio for CDN upload
  (byte-exact match), removes broken slug fallback, syncs all episode
  media to CDN after publishing
- Fix f-string syntax error in publish_episode.py (Python <3.12)
- Enable CORS on BunnyCDN pull zone for json files
- CDN URLs for website OG images, stem recorder bug fixes, LLM token
  budget tweaks, session context in CLAUDE.md

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

backend/services/llm.py

@@ -154,7 +154,7 @@ class LLMService:
                 json={
                     "model": model,
                     "messages": messages,
-                    "max_tokens": max_tokens or 150,
+                    "max_tokens": max_tokens or 300,
                     "temperature": 0.8,
                     "top_p": 0.92,
                     "frequency_penalty": 0.5,