feat(downloads): Logs button on failing-sources rollup rows

Operator-asked 2026-06-01: each row in the "X sources are failing"
rollup needs a button to surface the last run's stdout/stderr/error
without leaving the Downloads tab to find the matching event row.

Wired:
- `downloadsStore.loadLastForSource(sourceId)` two-steps via
  `GET /api/downloads?source_id=N&limit=1` (most recent event) then
  the existing `loadOne(id)` for the full detail payload. Returns
  null if no events exist (toast warns).
- `FailingSourcesCard` row: new text button `Logs` with
  `mdi-text-box-search-outline`, per-row spinner via `logLoadingIds`,
  emits `view-logs` with the source record.
- `DownloadsTab.onViewFailingLogs` is the handler — same
  `DownloadDetailModal` instance the event-row clicks use.

frontend/src/components/subscriptions/DownloadsTab.vue

@@ -58,6 +58,7 @@
       :retrying-all="retryingAll"
       @retry="onRetrySource"
       @retry-all="onRetryAll"
+      @view-logs="onViewFailingLogs"
     />
 
     <div v-if="store.loading && store.events.length === 0" class="fc-dl__loading">
@@ -364,6 +365,23 @@ watch(filterModel, async (m) => {
 async function openDetail(id) {
   await store.loadOne(id)
 }
+
+async function onViewFailingLogs(source) {
+  // Find and open the most recent DownloadEvent for this source.
+  // Reuses the existing DownloadDetailModal — same stdout/stderr/error
+  // surface the row-click in the events feed shows.
+  try {
+    const ev = await store.loadLastForSource(source.id)
+    if (!ev) {
+      toast({
+        text: `No download events recorded for ${source.artist_name || source.platform} yet.`,
+        type: 'warning',
+      })
+    }
+  } catch (e) {
+    toast({ text: `Failed to load logs: ${e.message}`, type: 'error' })
+  }
+}
 </script>
 
 <style scoped>