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

internal/static/js/app.js

@@ -378,6 +378,19 @@ async function deleteItem(id) {
 	}
 }
 
+// Play video (initial click to start)
+function playVideo(overlay) {
+	const container = overlay.parentElement;
+	const video = container.querySelector('video');
+	if (video) {
+		video.play();
+		overlay.remove();
+		// Show controls on hover
+		video.addEventListener('mouseenter', () => video.controls = true);
+		video.addEventListener('mouseleave', () => video.controls = false);
+	}
+}
+
 // Utility
 function escapeHtml(str) {
 	const div = document.createElement('div');