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/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,