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/lib/components/AlbumPanel.svelte

@@ -75,6 +75,13 @@
 			.catch((e) => (error = (e as Error).message));
 	});
 
+	$effect(() => {
+		// Keep the local album in sync with cover changes made elsewhere
+		// (context menu, photo panel) that go through the albums store.
+		const updated = albums.list.find((a) => a.id === albumId);
+		if (updated) album = updated;
+	});
+
 	$effect(() => {
 		const id = photoId;
 		if (!id) {

frontend/src/lib/components/PhotoPanel.svelte

@@ -6,6 +6,7 @@
 	import PanelSection from './PanelSection.svelte';
 	import PhotoInfoSettingsDialog from './PhotoInfoSettingsDialog.svelte';
 	import { auth } from '$lib/stores/auth.svelte';
+	import { albums } from '$lib/stores/albums.svelte';
 	import { transfer } from '$lib/stores/transfer.svelte';
 	import { viewerActions } from '$lib/stores/viewer-actions.svelte';
 	import { buildExifRows, EXIF_BASE, EXIF_CATALOG, EXIF_LABELS } from '$lib/exif';
@@ -179,6 +180,14 @@
 		}
 	}
 
+	async function setAsCover() {
+		try {
+			await albums.setCover(photo.album_id, photo.id);
+		} catch {
+			/* ignore */
+		}
+	}
+
 	async function confirmDelete() {
 		deleting = true;
 		try {
@@ -273,6 +282,12 @@
 				<Icon name="content_copy" class="size-4" />
 				Копировать
 			</button>
+			{#if canEdit}
+				<button type="button" class={navLink} onclick={setAsCover}>
+					<Icon name="wallpaper" class="size-4" />
+					Установить как обложку альбома
+				</button>
+			{/if}
 			{#if canDelete}
 				<button
 					type="button"