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>

website/css/style.css

@@ -218,6 +218,7 @@ a:hover {
 .btn-spotify { background: #1DB954; }
 .btn-youtube { background: #FF0000; }
 .btn-apple { background: #A033FF; }
+.btn-discord { background: #5865F2; }
 .btn-rss { background: #f26522; }
 
 /* Episodes */