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/css/app.css

@@ -296,6 +296,40 @@ button, input, textarea, select {
 	width: 100%;
 }
 
+.video-container {
+	position: relative;
+}
+
+.video-overlay {
+	position: absolute;
+	inset: 0;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	background: rgba(0, 0, 0, 0.3);
+	cursor: pointer;
+	transition: background 0.15s ease;
+}
+
+.video-overlay:hover {
+	background: rgba(0, 0, 0, 0.4);
+}
+
+.video-overlay .play-button {
+	width: 4rem;
+	height: 4rem;
+	background: rgba(0, 0, 0, 0.7);
+	color: #fff;
+	border-radius: 50%;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	font-size: 1.5rem;
+	padding-left: 0.25rem;
+}
+
+
+
 .embed-container {
 	position: relative;
 }