feat(subscriptions): dry-run backfill preview — B4 preview (plan #708)

Owning the walk lets an operator gauge "is this source worth a backfill?" before
arming one. ingest_core.Ingester.preview walks the first few feed pages and
counts media NOT already in the seen/dead ledgers, downloading nothing
(read-only). download_backends.preview_source resolves the campaign id + runs it
(native-only, mirrors verify_source_credential / run_download); POST
/api/sources/{id}/preview returns {total_new, posts_scanned, has_more, sample[]}
(409 on auth/drift/unresolvable, 400 for gallery-dl platforms). PatreonClient
gains post_meta(post) for the sample's title/date.

UI: a Patreon-only Preview button (mdi-eye-outline) on SourceRow + SourceCard
opens PreviewDialog — self-fetches with loading / error / empty / result states
and a "Start backfill" shortcut. Store action previewSource.

Tests: preview counts new media without downloading + samples only posts with
new items; page_limit caps the walk + flags has_more.

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

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -176,6 +176,7 @@
                     @check="onCheck"
                     @backfill="onBackfill"
                     @recover="onRecover"
+                    @preview="onPreview"
                   />
                   <tr v-if="item.sources.length === 0">
                     <td colspan="8" class="fc-subs__sources-empty">
@@ -253,6 +254,7 @@
             @check="onCheck"
             @backfill="onBackfill"
             @recover="onRecover"
+            @preview="onPreview"
           />
           <div v-if="item.sources.length === 0" class="fc-subs__sources-empty">
             No sources yet. Tap + to add one.
@@ -268,6 +270,11 @@
       @saved="onSourceSaved"
     />
     <ArtistCreateDialog v-model="showArtistDialog" @created="onArtistCreated" />
+    <PreviewDialog
+      v-model="showPreviewDialog"
+      :source="previewTarget"
+      @backfill="onPreviewBackfill"
+    />
   </div>
 </template>
 
@@ -284,6 +291,7 @@ import SourceCard from './SourceCard.vue'
 import SourceHealthDot from './SourceHealthDot.vue'
 import SourceFormDialog from './SourceFormDialog.vue'
 import ArtistCreateDialog from './ArtistCreateDialog.vue'
+import PreviewDialog from './PreviewDialog.vue'
 import PlatformChip from './PlatformChip.vue'
 import SchedulerStatusBar from './SchedulerStatusBar.vue'
 import { formatRelative } from '../../utils/date.js'
@@ -334,6 +342,8 @@ const showSourceDialog = ref(false)
 const editingSource = ref(null)
 const editingArtist = ref(null)
 const showArtistDialog = ref(false)
+const showPreviewDialog = ref(false)
+const previewTarget = ref(null)
 
 const artistFilter = computed(() => {
   const raw = route.query.artist_id
@@ -572,6 +582,18 @@ async function onRecover(source) {
   }
 }
 
+// Plan #708 B4: open the dry-run preview dialog (it self-fetches on open).
+function onPreview(source) {
+  previewTarget.value = source
+  showPreviewDialog.value = true
+}
+
+// "Start backfill" from inside the preview dialog → arm it + close.
+async function onPreviewBackfill(source) {
+  showPreviewDialog.value = false
+  await onBackfill(source)
+}
+
 async function checkAll(group) {
   let ok = 0
   let conflict = 0