fix(photos): faster large-album loading and album cover lifecycle

Large albums opened slowly and flickered; the default album cover was
also lost in the media-objects refactor.

Large-album loading:
- quantize signed object URLs within a window and send Cache-Control so
  previews are reused instead of re-fetched
- publish photo.updated (not photo.created) when variants are ready, so
  clients update in place without reordering the grid
- worker concurrency + newest-first variant priority; raise default batch

Album cover:
- first uploaded photo becomes the cover when none is set
- clear the cover when the cover photo is trashed or moved out
- "set as album cover" in the photo context menu and actions panel

OpenSpec changes: fix-large-album-load, fix-album-cover

frontend/src/routes/(app)/albums/[id]/+page.svelte

@@ -29,6 +29,7 @@
 	let uploader = $state<{ pick: () => void; upload: (files: FileList | File[]) => void }>();
 
 	let canUpload = $derived(['owner', 'admin', 'editor', 'member'].includes(role));
+	let canEdit = $derived(role === 'owner' || role === 'admin');
 
 	async function loadAlbum() {
 		const data = await api<{ album: Album; role: string }>(`/albums/${albumId}`);
@@ -100,6 +101,8 @@
 				break;
 			}
 			case 'photo.updated': {
+				// Variant readiness updates a photo already in the grid; never
+				// insert an unseen photo here, or the grid would reflow.
 				const photo = data as unknown as Photo;
 				photos = photos.map((p) => (p.id === photo.id ? photo : p));
 				break;
@@ -221,6 +224,15 @@
 							}
 						]
 					: []),
+				...(canEdit
+					? [
+							{
+								label: 'Установить как обложку альбома',
+								icon: 'wallpaper' as const,
+								run: () => albums.setCover(albumId, photo.id)
+							}
+						]
+					: []),
 				{
 					label: 'Переместить',
 					icon: 'arrow_back',