Add persistent caller voices, Discord, REC/on-air linking, SEO fixes, ep9

- Returning callers now keep their voice across sessions (stored in regulars.json)
- Backfilled voice assignments for all 11 existing regulars
- Discord button on homepage + link in all page footers
- REC and On-Air buttons now toggle together (both directions)
- Fixed host mic double-stream bug (stem_mic vs host_stream conflict)
- SEO: JSON-LD structured data on episode + how-it-works pages
- SEO: noscript fallbacks, RSS links, twitter meta tags
- Episode 9 transcript and sitemap update

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

backend/services/regulars.py

@@ -51,7 +51,7 @@ class RegularCallerService:
 
     def add_regular(self, name: str, gender: str, age: int, job: str,
                     location: str, personality_traits: list[str],
-                    first_call_summary: str) -> dict:
+                    first_call_summary: str, voice: str = None) -> dict:
         """Promote a first-time caller to regular"""
         # Retire oldest if at cap
         if len(self._regulars) >= MAX_REGULARS:
@@ -67,6 +67,7 @@ class RegularCallerService:
             "job": job,
             "location": location,
             "personality_traits": personality_traits,
+            "voice": voice,
             "call_history": [
                 {"summary": first_call_summary, "timestamp": time.time()}
             ],