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

backend/app/services/source_service.py

@@ -18,6 +18,7 @@ from ..models import (
     Source,
 )
 from .db_helpers import failing_sources_clause
+from .download_backends import uses_native_ingester
 from .gallery_dl import ErrorType
 from .membership_reconcile import KEPT_KEY, STOPPED_KEY
 from .membership_roster import gated_reasons_for_sources
@@ -125,6 +126,11 @@ class SourceRecord:
             "backfill_posts": self.backfill_posts,
             "tier_gated_count": self.tier_gated_count,
             "gated_reason": self.gated_reason,
+            # Recover / recapture exist only on the native ingester. Sent so the
+            # UI asks the backend's own predicate instead of keeping a copy of
+            # the platform list — the copy said "patreon, subscribestar" for a
+            # day after Discord went native (milestone 428).
+            "native_ingester": uses_native_ingester(self.platform),
         }
 
 
@@ -551,8 +557,8 @@ class SourceService:
         whole source); the two flags are mutually exclusive, so arming recapture
         clears bypass_seen. Clears prior cursor/chunk/stall state so it walks
         fresh from the top. The flag is cleared on completion (download_service)
-        and on stop. Recapture is Patreon-only (the native ingester's post-record
-        capture); inert elsewhere. The UI gates the action to Patreon sources."""
+        and on stop. Recapture needs the native ingester's post-record capture,
+        so the UI offers it on native sources only (`native_ingester`)."""
         source = (await self.session.execute(
             select(Source).where(Source.id == source_id)
         )).scalar_one_or_none()