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

tests/test_discord_downloader.py

@@ -0,0 +1,135 @@
+"""The native Discord downloader lands files where gallery-dl did (#4414).
+
+A cutover that names one file differently re-downloads it and imports a
+duplicate, so these pin the path, the name cleaning and the sidecar pairing
+against what gallery-dl's config produces — and that the records it writes read
+back through `parse_sidecar` as the same post the gallery-dl sidecars made.
+"""
+
+from __future__ import annotations
+
+import json
+
+from backend.app.services.discord_client import DiscordClient
+from backend.app.services.discord_downloader import (
+    DiscordDownloader,
+    channel_dir,
+    gdl_clean,
+    media_stem,
+)
+from backend.app.utils.sidecar import find_sidecar, parse_sidecar
+
+_PNG = b"\x89PNG\r\n\x1a\n" + b"\x00" * 16 + b"\x00\x00\x00\x00IEND\xaeB`\x82"
+
+
+class _Resp:
+    status_code = 200
+    headers: dict = {}
+
+    def raise_for_status(self):
+        pass
+
+    def iter_content(self, chunk_size=None):
+        yield _PNG
+
+
+class _Media:
+    def __init__(self):
+        self.calls = []
+
+    def get(self, url, stream=None, timeout=None, headers=None):
+        self.calls.append(url)
+        return _Resp()
+
+
+def _message(**extra):
+    return {
+        "id": "1234", "type": 0, "timestamp": "2026-09-20T23:30:00.000000+00:00",
+        "content": "new set!", "channel_id": "22",
+        "author": {"id": "7", "username": "artist"},
+        "attachments": [{"url": "https://cdn.discordapp.com/attachments/2/3/Red%3AAlt.PNG?ex=1"}],
+        "embeds": [],
+        "_meta": {"server": "Studio", "server_id": "11", "channel": "drops/nsfw",
+                  "channel_id": "22", "is_thread": False},
+        **extra,
+    }
+
+
+def _downloader(tmp_path, session=None):
+    return DiscordDownloader(tmp_path, validate=False, session=session or _Media())
+
+
+def test_names_follow_gallery_dls_pattern_and_linux_cleaning(tmp_path):
+    msg = _message()
+    [media] = DiscordClient.extract_media(msg)
+    assert channel_dir(tmp_path, "art", msg) == tmp_path / "art" / "discord" / "drops_nsfw"
+    # `:` survives: gallery-dl on Linux only replaces `/`.
+    assert media_stem(msg, media) == "20260920_1234_01_Red:Alt"
+    assert gdl_clean("a\x07b/c") == "ab_c"
+
+
+def test_a_channel_with_no_name_adds_no_directory(tmp_path):
+    msg = _message(_meta={"channel": "  "})
+    assert channel_dir(tmp_path, "art", msg) == tmp_path / "art" / "discord"
+
+
+def test_a_file_gallery_dl_already_wrote_is_not_fetched_again(tmp_path):
+    msg = _message()
+    media = DiscordClient.extract_media(msg)
+    existing = tmp_path / "art" / "discord" / "drops_nsfw" / "20260920_1234_01_Red:Alt.png"
+    existing.parent.mkdir(parents=True)
+    existing.write_bytes(_PNG)
+    session = _Media()
+    [out] = _downloader(tmp_path, session).download_post(msg, media, "art")
+    assert out.status == "skipped_disk" and out.path == existing
+    assert session.calls == []
+
+
+def test_a_new_file_gets_a_sidecar_the_importer_pairs_to_its_message(tmp_path):
+    msg = _message()
+    [out] = _downloader(tmp_path).download_post(msg, DiscordClient.extract_media(msg), "art")
+    assert out.status == "downloaded"
+    assert out.path.name == "20260920_1234_01_Red:Alt.png"
+    sidecar = find_sidecar(out.path)
+    assert sidecar is not None
+    data = json.loads(sidecar.read_text())
+    # No `id`/`post_id`: either would outrank message_id as the post id.
+    assert "id" not in data and "post_id" not in data
+    sd = parse_sidecar(data)
+    assert sd.external_post_id == "1234"
+    assert sd.source_url.startswith("https://cdn.discordapp.com/")
+
+
+def test_a_seen_file_is_skipped_without_a_request(tmp_path):
+    msg = _message()
+    session = _Media()
+    [out] = _downloader(tmp_path, session).download_post(
+        msg, DiscordClient.extract_media(msg), "art", is_seen=lambda m: True,
+    )
+    assert out.status == "skipped_seen" and session.calls == []
+
+
+def test_the_message_record_reads_back_as_the_gallery_dl_post(tmp_path):
+    rec = _downloader(tmp_path).write_post_record(_message(), "art")
+    assert rec.path.name == "20260920_1234_post.json"
+    assert rec.body_chars == len("new set!")
+    sd = parse_sidecar(json.loads(rec.path.read_text()))
+    assert sd.platform == "discord"
+    assert sd.external_post_id == "1234"
+    assert sd.post_url == "https://discord.com/channels/11/22/1234"
+    assert sd.description == "new set!"
+    assert sd.post_date.isoformat().startswith("2026-09-20T23:30")
+
+
+def test_the_record_is_not_a_media_sidecar(tmp_path):
+    """It must not pair with any file of the message."""
+    msg = _message()
+    dl = _downloader(tmp_path)
+    [out] = dl.download_post(msg, DiscordClient.extract_media(msg), "art")
+    rec = dl.write_post_record(msg, "art")
+    assert find_sidecar(out.path) != rec.path
+
+
+def test_an_empty_re_read_never_blanks_a_stored_body(tmp_path):
+    rec = _downloader(tmp_path).write_post_record(_message(content=""), "art", revisit=True)
+    assert rec.path is None