refactor: the native downloaders share one download loop, file record and seen-ledger key; a clean walk marks its credential verified

Patreon, SubscribeStar and Discord each carried their own copy of the same
per-post download loop, file record and seen-ledger key, and every copy was
another place a fix could be missed.

- BaseNativeDownloader runs one post's files: the seen and on-disk skips,
  the mid-post time-box, pacing, validation and the per-file sidecar. Each
  platform overrides only where its layout differs: Discord's channel folder,
  file names and message-id sidecar, and Patreon's Mux video through yt-dlp.
- One MediaItem and one ledger_key in native_ingest_common replace the three
  per-client copies. Keys are unchanged, except that a SubscribeStar file with
  no gallery id is now keyed by its filename instead of an empty id.
- DownloadResult, ErrorType and the run-stats and tier-gated helpers move
  from gallery_dl.py, which now serves only HentaiFoundry, to
  download_result.py. The unused delegator methods on GalleryDLService go.
- The download lock's platform list is the native-ingester list, not a copy.
- A walk that used a credential and finished with no error type stamps that
  credential's last-verified time, so the credentials page shows when the
  account last worked. A tier-limited walk does not count: an expired session
  hides paid posts the same way.
- The extension no longer clears a retired pixiv token on startup.
- Two comments that still described every download as gallery-dl now name
  both backends.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/platform_lock.py

@@ -17,16 +17,16 @@ import logging
 import redis
 
 from ..config import get_config
+from .download_backends import NATIVE_INGESTER_PLATFORMS
 
 log = logging.getLogger(__name__)
 
-# Platforms walked one-at-a-time. gallery-dl platforms are intentionally NOT
-# here: each runs as a self-pacing subprocess and they're lower-volume. The
-# native-ingester platforms are serialized (one paced scrape/API walk at a time).
-# Add a platform here to cap it to a single concurrent walk. Discord most of
-# all: every source walks on the operator's ONE user token, and parallel walks
+# Platforms walked one-at-a-time: every native-ingester platform, one paced
+# scrape/API walk at a time. gallery-dl platforms are not: each runs as a
+# self-pacing subprocess and they are lower-volume. Discord most of all needs
+# it: every source walks on the operator's ONE user token, and parallel walks
 # on a user account are both how its rate limit trips and what gets it flagged.
-SERIALIZED_PLATFORMS = frozenset({"patreon", "subscribestar", "discord"})
+SERIALIZED_PLATFORMS = NATIVE_INGESTER_PLATFORMS
 
 _LOCK_PREFIX = "fc:download_lock:"