feat(dashboards): 1600px max-width, richer Downloads filters, needs-attention + sticky headers

Operator-flagged 2026-05-28: the Subscriptions/Downloads dashboards were
full-bleed and thin on filtering. Chosen via AskUserQuestion.

**Layout**
- SubscriptionsView capped at a centered max-width 1600px (covers all
  three subtabs) so rows aren't a mile wide on ultrawide monitors.
- Sticky control headers on both tabs (top: 48px, below the top nav,
  opaque bg) so filters/stat-chips stay reachable while scrolling a
  long list.

**Downloads filters (all four requested)**
- Stat chips are now clickable filters: click Queued/Running/Completed/
  Failed/Skipped to filter the list to that status; the active chip is
  outlined + elevated; re-click clears.
- Free-text search box over the loaded events (artist / platform /
  error substring).
- Artist filter: the filter popover's numeric "Source ID" field is
  replaced with an artist autocomplete (sources.autocompleteArtist →
  artist_id, which /api/downloads already supports). Pill shows the
  artist name.
- "Show no-change scans" toggle (default OFF): hides status=ok/skipped
  rows with 0 files (the scheduled scans that found nothing) so real
  downloads + failures stand out.

**Subscriptions**
- "Needs attention" quick-filter chip: one click to show only artists
  with sources that have errors OR have never been checked; chip shows
  the count and disables the status dropdown while active.

Frontend-only — backend filter params (status/artist_id/date) and the
/api/downloads endpoint already supported everything.

frontend/src/views/SubscriptionsView.vue

@@ -1,5 +1,5 @@
 <template>
-  <v-container fluid class="pt-2 pb-6">
+  <v-container fluid class="pt-2 pb-6 fc-subs-shell">
     <v-tabs
       v-model="tab"
       align-tabs="start"
@@ -65,6 +65,12 @@ watch(() => route.query.tab, (q) => {
 </script>
 
 <style scoped>
+/* Cap the dashboards at a centered 1600px so rows aren't a mile wide on
+   wide/ultrawide monitors (operator-flagged 2026-05-28). */
+.fc-subs-shell {
+  max-width: 1600px;
+  margin-inline: auto;
+}
 .fc-subs-tabs {
   border-bottom: 1px solid rgb(var(--v-theme-on-surface-variant) / 0.18);
 }