feat(import-resilience L2): one-shot re-download for corrupt downloaded files

Layer 2 — remediate a corrupt file by re-fetching a fresh copy from its
source, bounded to a single attempt. Operator-requested 2026-05-28.

New backend/app/services/refetch_service.py:
- resolve_refetch_source: parse the failed file's sidecar → platform,
  derive the artist from the import path, find an ENABLED Source with a
  real feed URL for (artist, platform). Returns None for filesystem-only
  imports, missing sidecars, or `sidecar:<platform>:<slug>` synthetic
  anchors (not pollable).
- attempt_refetch: if not already refetched AND a Source resolves,
  delete the corrupt file (so gallery-dl's skip_existing re-fetches it),
  set ImportTask.refetched=True, and trigger ONE download_source
  re-check. Bounded by `refetched` so source-side corruption can't loop.

Wiring:
- Manual endpoint POST /api/import/tasks/<id>/refetch (only on 'failed'
  tasks). Returns refetch_queued / no_source / already_refetched /
  not_found / not_failed.
- Auto path in recover_interrupted_tasks: for each poison-pill row, if
  env FC_AUTO_REFETCH_CORRUPT=1, attempt_refetch (default OFF — the
  manual button is the primary path; auto is opt-in since re-fetch
  deletes a file + re-runs the downloader).
- Frontend: a cloud-refresh icon button on failed rows in ImportTaskList
  → stores.import.refetchTask → toast keyed on the result status.

Filesystem imports with no upstream return no_source — the operator's
only remediation there is replacing the file on disk, surfaced clearly
in the toast.

Tests: 404 unknown task, 400 non-failed task, no_source when
unresolvable, and the full resolvable-source path (file deleted,
refetched flag set, one download_source dispatched, second call is a
no-op). The resolvable test repoints the migration-seeded
import_settings(id=1) scan path rather than inserting a conflicting row.

frontend/src/components/settings/ImportTaskList.vue

@@ -51,6 +51,19 @@
           title="Click for full error"
         >{{ shorten(item.error, 60) }}</button>
       </template>
+      <template #item.actions="{ item }">
+        <v-btn
+          v-if="item.status === 'failed'"
+          icon size="x-small" variant="text"
+          :loading="refetching === item.id"
+          @click="onRefetch(item)"
+        >
+          <v-icon size="small">mdi-cloud-refresh</v-icon>
+          <v-tooltip activator="parent" location="top">
+            Re-fetch original (re-download from source)
+          </v-tooltip>
+        </v-btn>
+      </template>
     </v-data-table-virtual>
     <div v-if="store.hasMore" class="d-flex justify-center py-3">
       <v-btn variant="text" size="small" @click="onLoadMore">Load more</v-btn>
@@ -149,9 +162,29 @@ const headers = [
   { title: 'Source', key: 'source_path', sortable: false },
   { title: 'Size', key: 'size_bytes', sortable: false, width: 90 },
   { title: 'Created', key: 'created_at', sortable: false, width: 150 },
-  { title: 'Note', key: 'error', sortable: false }
+  { title: 'Note', key: 'error', sortable: false },
+  { title: '', key: 'actions', sortable: false, width: 56 }
 ]
 
+const refetching = ref(null)
+const _REFETCH_MSG = {
+  refetch_queued: { text: 'Re-fetch queued — re-downloading from source', type: 'success' },
+  no_source: { text: 'No re-fetchable source (filesystem import — replace the file manually)', type: 'info' },
+  already_refetched: { text: 'Already re-fetched once', type: 'info' },
+}
+async function onRefetch(item) {
+  refetching.value = item.id
+  try {
+    const res = await store.refetchTask(item.id)
+    const msg = _REFETCH_MSG[res.status] || { text: `Re-fetch: ${res.status}`, type: 'info' }
+    window.__fcToast?.(msg)
+  } catch (e) {
+    window.__fcToast?.({ text: `Re-fetch failed: ${e.message}`, type: 'error' })
+  } finally {
+    refetching.value = null
+  }
+}
+
 const hasFailed = computed(() => store.tasks.some(t => t.status === 'failed'))
 const hasStuck = computed(() => store.tasks.some(
   t => t.status === 'pending' || t.status === 'queued' || t.status === 'processing'