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/api/sources.py

@@ -230,52 +230,6 @@ async def set_backfill(source_id: int):
     return jsonify(record.to_dict())
 
 
-@sources_bp.route("/<int:source_id>/preview", methods=["POST"])
-async def preview_source_endpoint(source_id: int):
-    """Plan #708 B4: dry-run — count what a backfill WOULD download for a native
-    platform (Patreon today), without downloading. Walks the first few feed pages
-    and counts media not already in the seen/dead ledgers. Returns
-    {total_new, posts_scanned, pages_scanned, has_more, sample[]} or 409 + reason
-    (unresolvable campaign id / auth / drift). 400 for gallery-dl platforms (no
-    cheap dry-run — their verify is a slow --simulate)."""
-    from pathlib import Path
-
-    from ..services.credential_service import CredentialService
-    from ..services.download_backends import preview_source, uses_native_ingester
-    from ..tasks._sync_engine import sync_session_factory
-    from .credentials import _get_crypto
-
-    async with get_session() as session:
-        rec = await SourceService(session).get(source_id)
-        if rec is None:
-            return _bad("not_found", status=404)
-        if not uses_native_ingester(rec.platform):
-            return _bad(
-                "unsupported",
-                detail="Preview is only available for native-ingester platforms.",
-                status=400,
-            )
-        cred = CredentialService(session, _get_crypto())
-        cookies_path = await cred.get_cookies_path(rec.platform)
-        auth_token = await cred.get_token(rec.platform)
-
-    # The walk + ledger reads are sync (run off the request loop); the process
-    # sync engine is the same one the download task uses.
-    result = await preview_source(
-        platform=rec.platform,
-        url=rec.url,
-        source_id=source_id,
-        config_overrides=rec.config_overrides or {},
-        cookies_path=str(cookies_path) if cookies_path else None,
-        images_root=Path("/images"),
-        sync_session_factory=sync_session_factory(),
-        auth_token=auth_token,
-    )
-    if "error" in result:
-        return _bad("preview_failed", detail=result["error"], status=409)
-    return jsonify(result)
-
-
 @sources_bp.route("/<int:source_id>/check", methods=["POST"])
 async def check_source(source_id: int):
     """FC-3c: enqueue a download for this source.

backend/app/services/download_backends.py

@@ -24,7 +24,6 @@ import asyncio
 from pathlib import Path
 
 from .gallery_dl import DownloadResult, ErrorType
-from .native_ingest_common import NativeIngestError
 from .patreon_ingester import PatreonIngester
 from .patreon_resolver import extract_vanity, resolve_campaign_id_for_source
 from .pixiv_client import user_id_from_url
@@ -204,49 +203,6 @@ async def _run_native_ingester(
     return dl_result, resolved_campaign_id
 
 
-async def preview_source(
-    *,
-    platform: str,
-    url: str,
-    source_id: int,
-    config_overrides: dict | None,
-    cookies_path: str | None,
-    images_root: Path,
-    sync_session_factory,
-    auth_token: str | None = None,
-    page_limit: int = 3,
-) -> dict:
-    """Dry-run preview for a native platform (plan #708 B4): resolve the campaign
-    id, then walk a few pages counting media not already seen/dead — no download.
-
-    Returns the preview dict (total_new / posts_scanned / pages_scanned /
-    has_more / sample), or `{"error": msg}` on a resolve / auth / drift failure.
-    Native-only — the caller gates on `uses_native_ingester`.
-    """
-    import asyncio
-
-    campaign_id, _ = await _resolve_native_campaign_id(
-        platform, url, cookies_path, config_overrides or {}
-    )
-    if not campaign_id:
-        return {"error": _campaign_resolution_error(platform, url)}
-    ingester = _native_ingester_cls(platform)(
-        images_root=images_root,
-        cookies_path=cookies_path,
-        session_factory=sync_session_factory,
-        auth_token=auth_token,
-    )
-    loop = asyncio.get_running_loop()
-    try:
-        result = await loop.run_in_executor(
-            None,
-            lambda: ingester.preview(source_id, campaign_id, page_limit=page_limit),
-        )
-    except NativeIngestError as exc:
-        return {"error": f"Couldn't preview: {exc}"}
-    return result
-
-
 async def verify_source_credential(
     *,
     platform: str,

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: