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.