feat: finish the Discord switchover — recapture on every native source, backfills that run, gallery-dl's Discord config retired (milestone 428)

- Recover and Recapture show on every native source. The menu gated them on
  a copied platform list ('patreon', 'subscribestar') that went stale when
  Discord moved over. Sources now carry `native_ingester` from the backend's
  own predicate.
- A running backfill is due on every scheduler tick. Nothing queued a
  backfill's next chunk: each one waited for the source's regular interval,
  so an armed backfill sat idle until the next check (8h at the default) and
  a five-chunk walk took most of two days. The in-flight guard and the
  platform lock keep one chunk at a time. A failing source falls back to its
  backoff, and a stalled or out-of-budget walk stops being due. It also runs
  when the artist has auto-check off, since the operator started it by hand.
- gallery-dl no longer carries Discord: its naming constants, platform
  defaults, sidecar-mirroring postprocessor and token injection are gone.
  The naming test moves to the native downloader and still renders against
  the real gallery-dl sidecar fixture. That is the guard that the files
  gallery-dl wrote are found on disk rather than fetched again.

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

tests/test_discord_naming.py

@@ -0,0 +1,69 @@
+"""Discord file naming, checked against the metadata gallery-dl really emitted.
+
+Discord moved from gallery-dl to the native downloader in milestone 428. Every
+file gallery-dl wrote is on disk under gallery-dl's naming, and the native
+downloader finds it there — skips it instead of fetching it again — only if it
+builds the SAME name. This file used to pin gallery-dl's patterns (#3999: they
+named keys the extractor never set, and every file landed as
+`None/<date>_None_<name>`); it now pins the native downloader to what those
+patterns produced from the same real metadata.
+
+The fixture keeps the key set and value types of a real attachment sidecar
+from the operator's instance (2026-09-13), with every value invented.
+"""
+
+import json
+from pathlib import Path
+
+from backend.app.services.discord_client import DiscordClient
+from backend.app.services.discord_downloader import channel_dir, media_stem
+from backend.app.utils.sidecar import find_sidecar
+
+_FIXTURE = Path(__file__).parent / "fixtures" / "discord_attachment_sidecar.json"
+
+
+def _message_from_the_fixture():
+    """The API message that produced the fixture's sidecar."""
+    gdl = json.loads(_FIXTURE.read_text())
+    date, time = gdl["date"].split(" ")
+    return {
+        "id": gdl["message_id"],
+        "type": 0,
+        "timestamp": f"{date}T{time}.000000+00:00",
+        "content": gdl["message"],
+        "attachments": [{"id": "500000000000000005", "url": gdl["url"]}],
+        "embeds": [],
+        "_meta": {"channel": gdl["channel"], "channel_id": gdl["channel_id"],
+                  "server": gdl["server"], "server_id": gdl["server_id"]},
+    }
+
+
+def test_the_native_name_is_the_name_gallery_dl_wrote(tmp_path):
+    """gallery-dl's `{date:%Y%m%d}_{message_id}_{num:>02}_{filename}.{extension}`
+    in directory `{channel}`, rendered from the fixture."""
+    msg = _message_from_the_fixture()
+    [media] = DiscordClient.extract_media(msg)
+    assert media_stem(msg, media) + "." + media.extension == (
+        "20240716_300000000000000003_01_image.png"
+    )
+    assert channel_dir(tmp_path, "a", msg) == tmp_path / "a" / "discord" / "nsfw-drops"
+
+
+def test_a_gallery_dl_sidecar_on_disk_still_pairs_with_its_file(tmp_path):
+    """Files gallery-dl wrote keep their full sidecars beside them; the native
+    name must be the stem those sidecars were named after."""
+    msg = _message_from_the_fixture()
+    [media] = DiscordClient.extract_media(msg)
+    stem = media_stem(msg, media)
+    (tmp_path / f"{stem}.png").write_bytes(b"x")
+    (tmp_path / f"{stem}.json").write_text(_FIXTURE.read_text())
+    assert find_sidecar(tmp_path / f"{stem}.png") == tmp_path / f"{stem}.json"
+
+
+def test_two_files_with_the_same_original_name_get_distinct_names():
+    """`image.png` is Discord's default name; two messages must not collide."""
+    one = _message_from_the_fixture()
+    two = {**one, "id": "300000000000000099"}
+    [m1] = DiscordClient.extract_media(one)
+    [m2] = DiscordClient.extract_media(two)
+    assert media_stem(one, m1) != media_stem(two, m2)