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/stores/downloads.js

@@ -55,6 +55,21 @@ export const useDownloadsStore = defineStore('downloads', () => {
     return selected.value
   }
 
+  // Open the detail modal for the most recent DownloadEvent of a given
+  // source. Used by the failing-sources rollup's "Logs" button so the
+  // operator can troubleshoot without leaving the Downloads tab to find
+  // the row (operator-flagged 2026-06-01).
+  async function loadLastForSource(sourceId) {
+    const events = await api.get('/api/downloads', {
+      params: { source_id: sourceId, limit: 1 },
+    })
+    if (!events.length) {
+      selected.value = null
+      return null
+    }
+    return await loadOne(events[0].id)
+  }
+
   async function applyFilter(patch) {
     filter.value = { ...filter.value, ...patch }
     await loadFirst()
@@ -98,7 +113,8 @@ export const useDownloadsStore = defineStore('downloads', () => {
   return {
     events, cursor, hasMore, filter, selected, loading, error, stats,
     activity, failing, activeEvents,
-    loadFirst, loadMore, loadOne, applyFilter, closeDetail, loadStats,
+    loadFirst, loadMore, loadOne, loadLastForSource, applyFilter,
+    closeDetail, loadStats,
     loadActivity, loadFailing, loadActive, recoverStalled,
   }
 })