Add gallery admin and video media support.

This updates gallery handling to support video playback with generated poster thumbnails, adds authenticated admin upload/delete flows, and improves dev/runtime behavior including reliable thumbnail generation and media-safe response handling.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/internal/gallery/hero.go

@@ -19,12 +19,17 @@ func HeroRelPath() string {
 func SelectHero(images []Image) (Image, bool) {
 	want := HeroRelPath()
 	for _, img := range images {
+		if img.IsVideo {
+			continue
+		}
 		if img.RelPath == want {
 			return img, true
 		}
 	}
-	if len(images) > 0 {
-		return images[0], true
+	for _, img := range images {
+		if !img.IsVideo {
+			return img, true
+		}
 	}
 	return Image{}, false
 }