feat(credentials+downloads): real credential Verify button + live download-activity polling

Operator-flagged 2026-05-28, two asks.

**1. Credential Verify (was missing vs GS — and now actually verifies).**
GS's Verify was a stub (`TODO: implement actual verification` — just
stamped last_verified). FC does a real check, which matters given the
recent auth pain (subscribestar age cookie, HF host-only PHPSESSID):

- GalleryDLService.verify(url, platform, cookies_path, auth_token) runs
  gallery-dl in `--simulate --range 1-1` mode (no download) against the
  URL with the materialized credentials, then reuses _categorize_error:
  returncode 0 / NO_NEW_CONTENT → valid; AUTH_ERROR → invalid; other →
  inconclusive (reason surfaced). 45s timeout.
- POST /api/credentials/<platform>/verify picks an enabled Source for
  the platform to probe, runs verify, and on success stamps
  credential.last_verified (new CredentialService.mark_verified).
  Returns {valid: bool|null, reason, last_verified?}. valid=null means
  untestable (no credential, or no enabled source to point at).
- CredentialCard gains a Verify button (on credentialed cards) + a
  result chip (Verified ✓ / Failed / Untestable) and a toast with the
  reason. SettingsTab reloads on @verified so last_verified refreshes.

**2. Live download-activity feedback.** The Downloads tab was static —
no way to tell if downloads were succeeding without manually hitting
Refresh. It now auto-polls: stats every 4s, and the event list too
while anything is queued/running. Polling pauses when the tab is
backgrounded (document.hidden) and the list reload is skipped on idle
ticks to stay light. A pulsing "● live" indicator next to the stat
chips shows when auto-refresh is active (queued+running > 0); honors
prefers-reduced-motion.

Tests: verify endpoint — untestable with no credential, untestable with
no enabled source, valid+stamped on success (gallery-dl mocked), and
auth-failure reported without stamping.

frontend/src/components/subscriptions/DownloadsTab.vue

@@ -2,6 +2,10 @@
   <div>
     <div class="fc-dl__top">
       <DownloadStatChips :stats="store.stats" />
+      <span v-if="liveActive" class="fc-dl__live" title="Auto-refreshing while downloads are active">
+        <span class="fc-dl__live-dot" />
+        live
+      </span>
       <v-spacer />
       <v-btn variant="text" icon @click="refresh">
         <v-icon>mdi-refresh</v-icon>
@@ -74,7 +78,7 @@
 </template>
 
 <script setup>
-import { computed, onMounted, reactive, ref, watch } from 'vue'
+import { computed, onMounted, onUnmounted, reactive, ref, watch } from 'vue'
 import { useRoute } from 'vue-router'
 
 import { useDownloadsStore } from '../../stores/downloads.js'
@@ -105,12 +109,39 @@ async function refresh() {
   ])
 }
 
+// Live auto-refresh: while any download is queued or running, poll the
+// stats + first page every 4s so the operator can watch events succeed/
+// fail in real time without hitting Refresh. Polling stops automatically
+// when the queue drains (nothing pending/running), and is paused while
+// the tab is backgrounded. Operator-flagged 2026-05-28: couldn't tell if
+// downloads were succeeding because the view was static.
+const POLL_MS = 4000
+const liveActive = computed(
+  () => (store.stats.pending || 0) + (store.stats.running || 0) > 0,
+)
+let pollId = null
+function startPolling() {
+  if (pollId) return
+  pollId = setInterval(async () => {
+    if (document.hidden) return
+    // Refresh stats cheaply every tick; only reload the event list when
+    // there's active work to reflect (keeps idle ticks light).
+    await store.loadStats(24)
+    if (liveActive.value) await store.loadFirst()
+  }, POLL_MS)
+}
+function stopPolling() {
+  if (pollId) { clearInterval(pollId); pollId = null }
+}
+
 onMounted(() => {
   if (route.query.source_id) {
     filterModel.value = { ...filterModel.value, source_id: Number(route.query.source_id) }
   }
   refresh()
+  startPolling()
 })
+onUnmounted(stopPolling)
 
 // Client-side date filter on the loaded page (avoids a backend round-trip
 // for the date pickers; the existing /api/downloads endpoint can grow
@@ -194,6 +225,23 @@ async function openDetail(id) {
   flex-wrap: wrap;
 }
 .fc-dl__filter { margin-bottom: 12px; }
+.fc-dl__live {
+  display: inline-flex; align-items: center; gap: 5px;
+  font-size: 0.72rem; text-transform: uppercase; letter-spacing: 0.06em;
+  color: rgb(var(--v-theme-info));
+}
+.fc-dl__live-dot {
+  width: 7px; height: 7px; border-radius: 50%;
+  background: rgb(var(--v-theme-info));
+  animation: fc-dl-pulse 1.4s ease-in-out infinite;
+}
+@keyframes fc-dl-pulse {
+  0%, 100% { opacity: 1; transform: scale(1); }
+  50% { opacity: 0.35; transform: scale(0.7); }
+}
+@media (prefers-reduced-motion: reduce) {
+  .fc-dl__live-dot { animation: none; }
+}
 .fc-dl__loading, .fc-dl__empty {
   display: flex; justify-content: center; padding: 3rem 0;
   color: rgb(var(--v-theme-on-surface-variant));