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/models/__init__.py

@@ -8,6 +8,8 @@ from .backup_run import BackupRun
 from .base import Base
 from .character_prototype import CcipPrototypeState, CharacterPrototype
 from .credential import Credential
+from .discord_failed_media import DiscordFailedMedia
+from .discord_seen_media import DiscordSeenMedia
 from .download_event import DownloadEvent
 from .external_link import ExternalLink
 from .gpu_job import GpuJob
@@ -56,6 +58,8 @@ __all__ = [
     "BackupRun",
     "Source",
     "Credential",
+    "DiscordFailedMedia",
+    "DiscordSeenMedia",
     "PatreonFailedMedia",
     "PatreonSeenMedia",
     "SubscribeStarFailedMedia",

backend/app/models/discord_failed_media.py

@@ -0,0 +1,36 @@
+"""DiscordFailedMedia — per-source dead-letter ledger of Discord files that
+keep failing to download or validate.
+
+Mirror of SubscribeStarFailedMedia. After `attempts` reaches the dead-letter
+threshold a routine walk skips the file (recovery still retries it); a later
+clean download clears the row. `filehash` is the seen-ledger's key.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import ForeignKey, Integer, String, Text, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.types import DateTime
+
+from .base import Base
+
+
+class DiscordFailedMedia(Base):
+    __tablename__ = "discord_failed_media"
+    __table_args__ = (
+        UniqueConstraint("source_id", "filehash", name="uq_discord_failed_media_source_id"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    filehash: Mapped[str] = mapped_column(String(128), nullable=False)
+    attempts: Mapped[int] = mapped_column(Integer, nullable=False, default=1, server_default="1")
+    last_error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    first_failed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    last_failed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )

backend/app/models/discord_seen_media.py

@@ -0,0 +1,34 @@
+"""DiscordSeenMedia — per-source ledger of Discord files already downloaded.
+
+Mirror of SubscribeStarSeenMedia for the native Discord ingester (milestone
+428). `filehash` holds the ingester's per-file key, `<message_id>:<media_id>`:
+the attachment id, or for an embed a hash of its URL path. Not the file's
+position in the message — an edit that removes a file renumbers the rest
+(see `discord_client.MediaItem`). The message record's own gate is the
+synthetic `message:<id>` key in the same column.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import ForeignKey, Integer, String, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.types import DateTime
+
+from .base import Base
+
+
+class DiscordSeenMedia(Base):
+    __tablename__ = "discord_seen_media"
+    __table_args__ = (
+        UniqueConstraint("source_id", "filehash", name="uq_discord_seen_media_source_id"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    source_id: Mapped[int] = mapped_column(
+        ForeignKey("source.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    filehash: Mapped[str] = mapped_column(String(128), nullable=False)
+    post_id: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    seen_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )