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/handlers/item_page.go

@@ -53,9 +53,12 @@ func (c itemPageContent) Render(sw *ssr.Writer) error {
 		{{else if eq .Item.ItemType "embed"}}
 		{{if .Item.EmbedVideoURL}}
 		<div class="video-container">
-			<video controls poster="{{if .Item.ThumbnailID}}/media/{{.Item.ThumbnailID}}{{end}}">
-				<source src="{{.Item.EmbedVideoURL}}" type="video/mp4">
+			<video loop muted playsinline poster="{{if .Item.ThumbnailID}}/media/{{.Item.ThumbnailID}}{{end}}">
+				<source src="/proxy/video/{{.Item.ID}}" type="video/mp4">
 			</video>
+			<div class="video-overlay" onclick="playVideo(this)">
+				<span class="play-button">&#9654;</span>
+			</div>
 		</div>
 		{{else if .Item.ThumbnailID}}
 		<div class="image-container">