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/SourceActions.vue

@@ -34,16 +34,6 @@
         </v-btn>
       </template>
       <v-list density="compact" min-width="260">
-        <v-list-item
-          v-if="isPatreon && !running"
-          prepend-icon="mdi-eye-outline"
-          @click="emit('preview', source)"
-        >
-          <v-list-item-title>Preview backfill</v-list-item-title>
-          <v-list-item-subtitle>
-            Count what a backfill would download — nothing is downloaded
-          </v-list-item-subtitle>
-        </v-list-item>
         <v-list-item
           v-if="isPatreon && !running"
           prepend-icon="mdi-backup-restore"
@@ -75,7 +65,7 @@ const props = defineProps({
   source: { type: Object, required: true },
   checking: { type: Boolean, default: false },
 })
-const emit = defineEmits(['check', 'backfill', 'recover', 'preview', 'remove'])
+const emit = defineEmits(['check', 'backfill', 'recover', 'remove'])
 
 const running = computed(() => props.source.backfill_state === 'running')
 const recovering = computed(() => !!props.source.backfill_bypass_seen)