refactor(subscriptions): remove the dead backfill dry-run preview (#1281)

PreviewDialog.vue was orphaned — nothing mounted it — so the dry-run preview
was unreachable. Rather than wire it up, remove the whole chain: its unique
value (a capped 3-page count of what a backfill would grab) is low, and its
adjacent needs are already covered — auth validation by verify_source_credential,
and actually fetching recent posts by "Check now". Operator decision 2026-07-06.

Removed:
- frontend: PreviewDialog.vue + sources store previewSource()
- backend: POST /api/sources/<id>/preview route, download_backends.preview_source,
  IngestCore.preview() + its now-unused NativeIngestError import
- tests: the 3 ingester preview tests

Nothing else referenced the chain (verified). Shared campaign-resolution and
ledger helpers stay — they're used by run()/verify_source_credential.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/ingest_core.py

@@ -577,73 +577,6 @@ 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
-        # #874: same gated-post gate as run() — the preview must not count
-        # blurred locked-preview media as "new", or it would overstate a gated
-        # source's backlog (preview/apply parity, rule 93).
-        post_is_gated = getattr(self.client, "post_is_gated", None)
-        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
-            if post_is_gated and post_is_gated(post):
-                continue
-            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: