feat: Discord on the native core ingester — client, downloader, ledgers, wiring (milestone 428)

Discord was the last focus platform still on gallery-dl. This adds the native
path, mirrored from gallery-dl 1.32.13's discord extractor:

- discord_client: API v10 with the user token and gallery-dl's request
  profile (dated Firefox UA, Referer). Walks a server, category, forum,
  channel or thread in gallery-dl's order and pages each channel newest-first.
  Files are attachments, then embeds, then forwards, numbered across the
  message. The resume cursor is <channel>:<before>. Text-only messages are
  not posts, since gallery-dl never made them.
- discord_downloader: gallery-dl's on-disk layout, cleaned the way it cleans
  names on Linux (only `/` and control characters change), so existing files
  are skipped_disk rather than fetched again. Sidecars carry identity only.
  The message record keeps gallery-dl's keys, so parse_sidecar,
  derive_post_url and the drop grouping read it unchanged.
- The ledger keys on the attachment id (or a hash of an embed's URL path),
  not the file's position, which an edit can renumber. Migration 0111.
- DiscordIngester: token auth, body canary off (files-only drops are
  normal). Registered as native, verified by token, and serialised
  per-platform, since every source shares one user token.
- ingest_core: optional `skip_feed` client seam (#4413). A tick's early-out
  on a multi-channel source now ends the quiet channel, not the whole walk.
  Clients without the seam behave as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/platform_lock.py

@@ -23,8 +23,10 @@ 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.
-SERIALIZED_PLATFORMS = frozenset({"patreon", "subscribestar"})
+# 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
+# on a user account are both how its rate limit trips and what gets it flagged.
+SERIALIZED_PLATFORMS = frozenset({"patreon", "subscribestar", "discord"})
 
 _LOCK_PREFIX = "fc:download_lock:"