feat: cover retry backfill + admin force-resync for manga & chapter (0.50.0)

Adds a per-tick cover-backfill pass to the crawler daemon so mangas whose
cover download failed on first attempt get retried — the metadata pass's
early-stop optimisation otherwise prevents the walk from revisiting them.

Adds admin-only POST /admin/mangas/:id/resync and POST /admin/chapters/:id/resync
that refetch metadata + cover (or chapter content with force_refetch) from the
crawler source synchronously and return the refreshed row. Surfaced in the
UI as "Force resync" buttons on the manga detail and reader pages,
admin-only via session.user.is_admin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/manga/[id]/+page.svelte

@@ -1,13 +1,15 @@
 <script lang="ts">
-    import { fileUrl } from '$lib/api/client';
+    import { fileUrl, ApiError } from '$lib/api/client';
     import { createBookmark, deleteBookmark, type Bookmark } from '$lib/api/bookmarks';
     import {
         attachTag,
         detachTag,
         type AuthorRef,
         type GenreRef,
+        type MangaDetail,
         type TagRef
     } from '$lib/api/mangas';
+    import { resyncManga } from '$lib/api/admin';
     import { listTags, type Tag } from '$lib/api/tags';
     import { session } from '$lib/session.svelte';
     import Chip from '$lib/components/Chip.svelte';
@@ -16,9 +18,15 @@
     import FolderPlus from '@lucide/svelte/icons/folder-plus';
     import Pencil from '@lucide/svelte/icons/pencil';
     import UploadCloud from '@lucide/svelte/icons/upload-cloud';
+    import RefreshCw from '@lucide/svelte/icons/refresh-cw';
 
     let { data } = $props();
-    const manga = $derived(data.manga);
+    // `manga` is locally overridable so a successful force resync can
+    // swap in the refreshed detail (new cover URL, refreshed status,
+    // etc.) without a router reload. Falls back to the server-loaded
+    // data otherwise.
+    let mangaOverride = $state<MangaDetail | null>(null);
+    const manga = $derived<MangaDetail>(mangaOverride ?? data.manga);
     const chapters = $derived(data.chapters);
     const readProgress = $derived(data.readProgress);
     /** Chapter row from the local chapters list when present (so we
@@ -171,6 +179,31 @@
     const statusLabel = $derived(manga.status === 'completed' ? 'Completed' : 'Ongoing');
 
     let collectionModalOpen = $state(false);
+
+    // ---- Admin force resync ----
+    let resyncBusy = $state(false);
+    let resyncMessage = $state<{ kind: 'ok' | 'err'; text: string } | null>(null);
+    async function forceResync() {
+        if (!session.user?.is_admin || resyncBusy) return;
+        resyncBusy = true;
+        resyncMessage = null;
+        try {
+            const r = await resyncManga(manga.id);
+            mangaOverride = r.manga;
+            const coverNote = r.cover_fetched
+                ? ' Cover re-downloaded.'
+                : ' Cover unchanged.';
+            resyncMessage = {
+                kind: 'ok',
+                text: `Metadata ${r.metadata_status}.${coverNote}`
+            };
+        } catch (e) {
+            const msg = e instanceof ApiError ? e.message : (e as Error).message;
+            resyncMessage = { kind: 'err', text: msg };
+        } finally {
+            resyncBusy = false;
+        }
+    }
 </script>
 
 <svelte:head>
@@ -344,7 +377,34 @@
                         <UploadCloud size={16} aria-hidden="true" />
                         <span>Upload chapter</span>
                     </a>
+                    {#if session.user.is_admin}
+                        <button
+                            type="button"
+                            class="action"
+                            onclick={forceResync}
+                            disabled={resyncBusy}
+                            title="Refetch metadata + cover from the crawler source"
+                            data-testid="force-resync-manga"
+                        >
+                            <RefreshCw
+                                size={16}
+                                aria-hidden="true"
+                                class={resyncBusy ? 'spin' : ''}
+                            />
+                            <span>{resyncBusy ? 'Resyncing…' : 'Force resync'}</span>
+                        </button>
+                    {/if}
                 </div>
+                {#if resyncMessage}
+                    <p
+                        class="resync-msg"
+                        class:err={resyncMessage.kind === 'err'}
+                        role="status"
+                        data-testid="force-resync-message"
+                    >
+                        {resyncMessage.text}
+                    </p>
+                {/if}
             {:else}
                 <a class="action" href="/login" data-testid="bookmark-signin">
                     Sign in to bookmark or collect
@@ -586,6 +646,29 @@
         color: var(--text);
     }
 
+    .resync-msg {
+        margin-top: var(--space-2);
+        color: var(--text-muted);
+        font-size: var(--font-sm);
+    }
+
+    .resync-msg.err {
+        color: var(--danger);
+    }
+
+    :global(.spin) {
+        animation: spin 0.9s linear infinite;
+    }
+
+    @keyframes spin {
+        from {
+            transform: rotate(0deg);
+        }
+        to {
+            transform: rotate(360deg);
+        }
+    }
+
     .continue {
         display: flex;
         flex-direction: column;