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

alembic/versions/0111_discord_native_ledger.py

@@ -0,0 +1,75 @@
+"""Discord native ingester ledgers — seen and dead-letter, per source.
+
+Milestone 428, #4415. Discord moves off gallery-dl onto the native core, which
+keeps its memory of what a source has already fetched in these two tables
+instead of gallery-dl's archive. Same shape as the SubscribeStar pair.
+
+Revision ID: 0111
+Revises: 0110
+Create Date: 2026-09-24
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0111"
+down_revision = "0110"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        "discord_seen_media",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("source_id", sa.Integer(), nullable=False),
+        sa.Column("filehash", sa.String(length=128), nullable=False),
+        sa.Column("post_id", sa.String(length=64), nullable=True),
+        sa.Column(
+            "seen_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+        sa.ForeignKeyConstraint(
+            ["source_id"], ["source.id"],
+            name=op.f("fk_discord_seen_media_source_id_source"), ondelete="CASCADE",
+        ),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_discord_seen_media")),
+        sa.UniqueConstraint("source_id", "filehash", name="uq_discord_seen_media_source_id"),
+    )
+    op.create_index(
+        op.f("ix_discord_seen_media_source_id"), "discord_seen_media", ["source_id"],
+    )
+    op.create_table(
+        "discord_failed_media",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("source_id", sa.Integer(), nullable=False),
+        sa.Column("filehash", sa.String(length=128), nullable=False),
+        sa.Column("attempts", sa.Integer(), server_default="1", nullable=False),
+        sa.Column("last_error", sa.Text(), nullable=True),
+        sa.Column(
+            "first_failed_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+        sa.Column(
+            "last_failed_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+        sa.ForeignKeyConstraint(
+            ["source_id"], ["source.id"],
+            name=op.f("fk_discord_failed_media_source_id_source"), ondelete="CASCADE",
+        ),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_discord_failed_media")),
+        sa.UniqueConstraint(
+            "source_id", "filehash", name="uq_discord_failed_media_source_id",
+        ),
+    )
+    op.create_index(
+        op.f("ix_discord_failed_media_source_id"), "discord_failed_media", ["source_id"],
+    )
+
+
+def downgrade():
+    op.drop_index(op.f("ix_discord_failed_media_source_id"), table_name="discord_failed_media")
+    op.drop_table("discord_failed_media")
+    op.drop_index(op.f("ix_discord_seen_media_source_id"), table_name="discord_seen_media")
+    op.drop_table("discord_seen_media")