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_source_service.py

@@ -34,6 +34,24 @@ async def test_known_platforms_are_the_supported_four(db):
     assert "pixiv" not in KNOWN_PLATFORMS
 
 
+@pytest.mark.asyncio
+async def test_a_source_says_whether_it_is_on_the_native_ingester(db):
+    """The UI offers recover / recapture from this flag rather than a copied
+    platform list — the copy went stale when Discord moved (milestone 428)."""
+    artist = await _artist(db, name="Natty")
+    svc = SourceService(db)
+    discord = await svc.create(
+        artist_id=artist.id, platform="discord",
+        url="https://discord.com/channels/1/2",
+    )
+    hf = await svc.create(
+        artist_id=artist.id, platform="hentaifoundry",
+        url="https://www.hentai-foundry.com/user/Natty",
+    )
+    assert discord.to_dict()["native_ingester"] is True
+    assert hf.to_dict()["native_ingester"] is False
+
+
 @pytest.mark.asyncio
 async def test_create_flips_is_subscription_on_first_source(db):
     artist = await _artist(db)