Web UI: music channel, posted-to-Matrix glow, redesigned index, AVIF thumbnails

- Add music as a fourth channel (nav + theme color)
- Glowing themed border on cards that have been posted to Matrix
- Replace per-channel index sections with: "Pete just posted" strip,
  channel dashboard (last post, 24h count, totals), unified latest feed
- /img proxy: SSRF-guarded thumbnail re-encoder that resizes to 800px
  and runs avifenc -q 45, cached under data/img-cache

internal/web/server.go

@@ -34,6 +34,7 @@ var channels = []Channel{
 	{Slug: "gaming", Title: "Gaming", Theme: "gaming", Emoji: "🎮", Blurb: "Releases, platforms, and the people making the games."},
 	{Slug: "tech", Title: "Tech", Theme: "tech", Emoji: "💻", Blurb: "Industry, products, and the wires that hold it all together."},
 	{Slug: "politics", Title: "Politics", Theme: "politics", Emoji: "🏛️", Blurb: "Policy, power, and the news of the day."},
+	{Slug: "music", Title: "Music", Theme: "music", Emoji: "🎵", Blurb: "Records, scenes, and the artists shaping the sound."},
 }
 
 // Server is the HTTP server for Pete's web UI.
@@ -68,6 +69,7 @@ func New(cfg config.WebConfig) (*Server, error) {
 		return nil, err
 	}
 	mux.Handle("GET /static/", http.StripPrefix("/static/", http.FileServer(http.FS(staticSub))))
+	mux.HandleFunc("GET /img/{hash}.avif", s.handleImg)
 
 	mux.HandleFunc("GET /{$}", s.handleIndex)
 	for _, ch := range channels {