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>

backend/app/services/patreon_client.py

@@ -520,6 +520,18 @@ class PatreonClient:
 
         return _dedup_by_filehash(items)
 
+    @staticmethod
+    def post_meta(post: dict) -> dict:
+        """Title + published date for a post — for the preview sample (plan #708
+        B4). Part of the client contract `ingest_core.Ingester.preview` calls."""
+        attrs = post.get("attributes") or {}
+        title = attrs.get("title")
+        published = attrs.get("published_at")
+        return {
+            "title": title if isinstance(title, str) else None,
+            "date": published if isinstance(published, str) else None,
+        }
+
     # -- iteration ---------------------------------------------------------
 
     def iter_posts(