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/ingest_core.py

@@ -361,6 +361,67 @@ class Ingester:
             error_type=None, error_message=None,
         )
 
+    # -- preview (dry-run) -------------------------------------------------
+
+    def preview(
+        self,
+        source_id: int,
+        campaign_id: str,
+        *,
+        page_limit: int = 3,
+        sample_size: int = 10,
+    ) -> dict:
+        """Dry-run (plan #708 B4): walk up to `page_limit` pages and count media
+        NOT already in the seen/dead ledgers, WITHOUT downloading anything.
+
+        Read-only — only the seen/dead SELECTs touch the DB (short sessions). Lets
+        an operator gauge "is this source worth a backfill?" cheaply. Returns:
+            {total_new, posts_scanned, pages_scanned, has_more,
+             sample: [{title, date, new}, ...]}  # sample = posts with new media
+        A client-level failure (auth/drift) propagates to the caller.
+        """
+        total_new = 0
+        posts_scanned = 0
+        pages_scanned = 0
+        has_more = False
+        sample: list[dict] = []
+        unset = object()
+        last_page: object = unset
+        for post, included, page_cursor in self.client.iter_posts(
+            campaign_id, cursor=None
+        ):
+            if page_cursor != last_page:
+                last_page = page_cursor
+                pages_scanned += 1
+                if pages_scanned > page_limit:
+                    has_more = True
+                    pages_scanned = page_limit
+                    break
+            posts_scanned += 1
+            media = self.client.extract_media(post, included)
+            if not media:
+                continue
+            keys = [self._ledger_key(m) for m in media]
+            skip = self._seen_keys(source_id, keys) | self._dead_keys(source_id, keys)
+            new_count = sum(1 for m in media if self._ledger_key(m) not in skip)
+            total_new += new_count
+            if new_count > 0 and len(sample) < sample_size:
+                meta = self.client.post_meta(post)
+                sample.append(
+                    {
+                        "title": meta.get("title") or "(untitled)",
+                        "date": meta.get("date"),
+                        "new": new_count,
+                    }
+                )
+        return {
+            "total_new": total_new,
+            "posts_scanned": posts_scanned,
+            "pages_scanned": pages_scanned,
+            "has_more": has_more,
+            "sample": sample,
+        }
+
     # -- failure mapping (adapter overrides) -------------------------------
 
     def _failure_result(self, exc: Exception, _result) -> DownloadResult: