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/media.go

@@ -0,0 +1,36 @@
+package gallery
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+func isVideo(name string) bool {
+	switch strings.ToLower(filepath.Ext(name)) {
+	case ".mp4", ".webm", ".mov":
+		return true
+	default:
+		return false
+	}
+}
+
+func isGalleryMedia(name string) bool {
+	return isJPEG(name) || isVideo(name)
+}
+
+// thumbLookupRel is the path under thumbs/ (and hero/) used to find derivatives.
+// Videos use a JPEG poster: clip.mp4 → clip.jpg.
+func thumbLookupRel(rel string) string {
+	if isVideo(filepath.Base(rel)) {
+		return videoPosterRel(rel)
+	}
+	return rel
+}
+
+func videoPosterRel(rel string) string {
+	ext := filepath.Ext(rel)
+	if ext == "" {
+		return rel + ".jpg"
+	}
+	return strings.TrimSuffix(rel, ext) + ".jpg"
+}