Add video proxy to fix Firefox tracking protection blocking Twitter videos

Twitter's video.twimg.com CDN is blocked by Firefox's Enhanced Tracking
Protection when loaded cross-origin. This adds a server-side proxy that
streams videos through our domain.

- Add /proxy/video/{id} endpoint to proxy embed video URLs
- Update embed video player: click-to-play with overlay, muted, looping
- Show native controls on hover

cmd/server/main.go

@@ -128,6 +128,7 @@ func runWebServer(dbURL string, logger *slog.Logger) {
 
 	// Media
 	router.Handle("GET /media/{id}", handlers.HandleGetMedia)
+	router.Handle("GET /proxy/video/{id}", handlers.HandleProxyVideo)
 
 	// Auth API
 	router.Handle("POST /api/auth/login", handlers.HandleLogin)