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/scheduler_service.py

@@ -115,6 +115,32 @@ async def active_platform_cooldowns(session: AsyncSession) -> dict[str, datetime
     return active
 
 
+def backfill_ready(source: Source) -> bool:
+    """A deep walk the operator started, with budget left and no failure
+    backing it off — due NOW rather than at its next scheduled check.
+
+    A backfill runs one time-boxed chunk per download (plan #693), and nothing
+    queued the next chunk: each waited for the source's regular interval. At
+    the 8-hour default a freshly armed backfill sat untouched until the next
+    check (the operator armed one on 2026-09-25 and saw nothing happen) and a
+    five-chunk walk took most of two days. The tick's in-flight guard keeps
+    one chunk at a time per source and the platform lock one walk per
+    platform, so "due every tick" means "next chunk as soon as the last one
+    ends".
+
+    The failure gate is what keeps a broken source from retrying every
+    minute: any failed chunk raises `consecutive_failures`, which drops the
+    source back onto its backed-off interval. A chunk that fails to progress
+    twice marks the walk stalled (download_service), which ends it here too.
+    """
+    co = source.config_overrides or {}
+    return (
+        co.get("_backfill_state") == "running"
+        and (source.backfill_runs_remaining or 0) > 0
+        and not (source.consecutive_failures or 0)
+    )
+
+
 async def select_due_sources(session: AsyncSession) -> list[Source]:
     """Sources where (enabled, artist.auto_check) and now >= last_checked_at + effective_interval.
 
@@ -123,6 +149,9 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
     cooldown is the preventive half of the burst-prevention pair (per-source
     consecutive_failures backoff handles the offending source itself).
 
+    A running backfill (`backfill_ready`) is due on every tick, and whether
+    or not its artist is on auto-check — the operator started it by hand.
+
     Ordering: last_checked_at ASC NULLS FIRST, then id. Never-checked
     sources go first, then the longest-since-checked, so the most overdue
     sources hit Celery's FIFO download queue first. Anti-starvation: if
@@ -135,7 +164,6 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
         .options(selectinload(Source.artist))
         .join(Artist, Source.artist_id == Artist.id)
         .where(Source.enabled.is_(True))
-        .where(Artist.auto_check.is_(True))
         .order_by(Source.last_checked_at.asc().nulls_first(), Source.id)
     )).scalars().all()
 
@@ -147,6 +175,11 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
     for s in rows:
         if s.platform in cooldowns:
             continue
+        if backfill_ready(s):
+            due.append(s)
+            continue
+        if not s.artist.auto_check:
+            continue
         interval = compute_effective_interval(s, s.artist, settings)
         if s.last_checked_at is None:
             due.append(s)
@@ -161,6 +194,8 @@ def compute_next_check_at(
     source: Source, artist: Artist, settings: ImportSettings,
 ) -> datetime | None:
     """Return the projected datetime of the next check, or None if never checked."""
+    if backfill_ready(source):
+        return datetime.now(UTC)
     if source.last_checked_at is None:
         return None
     interval = compute_effective_interval(source, artist, settings)