feat(subs): subscriptions UX batch — error reasons, single-source rows, health sort, bulk backfill

Operator-requested follow-ups:
- #1 Failure reason on hover: the red error-count chip now shows source.last_error
  in a tooltip (desktop row + mobile card), so the cause (e.g. the new
  "vanity=cw" message) is visible without opening Downloads.
- #2 Collapse the single-source case: a subscription with exactly one source now
  shows that source's URL + its own actions (Check / Backfill / ⋮ / Edit) inline
  on the artist row — no expand needed for the common case. Multi-source keeps
  the artist-level actions + expandable per-source table.
- #3 Sort by health: the Health column is sortable on a numeric rank
  (never/ok/warn/fail) and the table defaults to worst-first, name as tiebreak.
- #4 Drop Preview: removed the low-value bounded-peek action from the menu and
  all its wiring (backend endpoint + store fn left in place, unused).
- #5 Backfill selected: a "Backfill" button in the bulk bar arms a run-until-done
  backfill on every enabled, not-already-running source in the selection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/SourceRow.vue

@@ -42,7 +42,12 @@
       <v-chip
         v-if="(source.consecutive_failures || 0) > 0"
         size="x-small" color="error" variant="tonal" label
-      >{{ source.consecutive_failures }}</v-chip>
+      >{{ source.consecutive_failures }}
+        <!-- #1: show the actual failure reason on hover instead of a bare count. -->
+        <v-tooltip v-if="source.last_error" activator="parent" location="top" max-width="480">
+          <span class="fc-source-row__err-text">{{ source.last_error }}</span>
+        </v-tooltip>
+      </v-chip>
       <v-chip
         v-else-if="source.backfill_state === 'running'"
         size="x-small" color="info" variant="tonal" label
@@ -66,7 +71,6 @@
         @check="$emit('check', $event)"
         @backfill="$emit('backfill', $event)"
         @recover="$emit('recover', $event)"
-        @preview="$emit('preview', $event)"
         @remove="$emit('remove', $event)"
       />
     </td>
@@ -83,7 +87,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover', 'preview'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })
@@ -127,6 +131,10 @@ function onToggleEnabled(value) {
   color: rgb(var(--v-theme-on-surface-variant));
   opacity: 0.6;
 }
+.fc-source-row__err-text {
+  white-space: pre-wrap;
+  word-break: break-word;
+}
 .fc-source-row__actions {
   white-space: nowrap;
   text-align: right;