Add cover URL edit button and modal

Rows without a cover (cover_url NULL or N/A) show a + button.
Clicking it opens a modal to enter a URL which is saved via new
PATCH /api/records/{id}/cover_url endpoint. cover_image is cleared
so n8n picks it up on the next backfill run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/MobileCards.vue

@@ -17,8 +17,13 @@ const props = defineProps<{
 const emit = defineEmits<{
   delete: [id: number]
   'toggle-seen': [record: MediaRecord]
+  'edit-cover': [record: MediaRecord]
 }>()
 
+function needsCover(record: MediaRecord): boolean {
+  return !record.cover_mime_type && (!record.cover_url || record.cover_url === 'N/A')
+}
+
 function onCardClick(record: MediaRecord, event: MouseEvent) {
   const target = event.target as HTMLElement
   if (target.closest('button, a, input, select, dl')) return
@@ -74,7 +79,15 @@ const detailFields: { key: keyof MediaRecord; label: string }[] = [
       @click="onCardClick(row, $event)"
     >
       <div class="card__hero">
-        <CoverImage :url="row.cover_url" :id="row.id" :has-cover-in-db="Boolean(row.cover_mime_type)" size="card" />
+        <div class="cover-wrap">
+          <CoverImage :url="row.cover_url" :id="row.id" :has-cover-in-db="Boolean(row.cover_mime_type)" size="card" />
+          <button
+            v-if="needsCover(row)"
+            class="cover-edit-btn"
+            title="Cover-URL eingeben"
+            @click.stop="emit('edit-cover', row)"
+          >＋</button>
+        </div>
         <div class="card__hero-text">
       <header class="card__header">
         <div class="card__meta">
@@ -153,6 +166,27 @@ const detailFields: { key: keyof MediaRecord; label: string }[] = [
   align-items: flex-start;
 }
 
+.cover-wrap {
+  position: relative;
+  flex-shrink: 0;
+}
+
+.cover-edit-btn {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  background: rgba(0, 0, 0, 0.45);
+  border: 2px dashed var(--border);
+  border-radius: var(--radius);
+  color: var(--text-muted);
+  font-size: 1.4rem;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
 .card__hero-text {
   flex: 1;
   min-width: 0;