feat(scheduler): order due sources by last_checked_at — most overdue first

select_due_sources returned rows in undefined order (Postgres-determined,
typically PK). At tick rates that outpace download-queue throughput, a
freshly-rerun source could keep getting re-queued ahead of one that's
still waiting for its first attempt this cycle. Operator-flagged
2026-05-30:

> if there are 8 hours before a source is due again and 40 full time
> downloads can happen in that period that means that there's a chance
> the first one to fire gets back into the download queue before item 41
> has a chance to get downloaded.

Added `ORDER BY last_checked_at ASC NULLS FIRST, id` to the due-source
SELECT. Never-checked sources go first, then longest-since-checked, then
ties broken by id. Combined with Celery's FIFO `download` queue, the
oldest-overdue source in each tick now reaches a worker before any
fresher one.

Test pins the ordering: a NULL-last_checked source, a 4-hour-overdue
source, and a 2-min-overdue source come back in that exact order from
select_due_sources.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/scheduler_service.py

@@ -116,6 +116,13 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
     whose platform is currently in a rate-limit cooldown are excluded — the
     cooldown is the preventive half of the burst-prevention pair (per-source
     consecutive_failures backoff handles the offending source itself).
+
+    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
+    queue throughput ever falls below the tick rate, a freshly-rerun source
+    can't keep cutting in line ahead of one that hasn't been checked at all.
+    Operator-confirmed 2026-05-30.
     """
     rows = (await session.execute(
         select(Source)
@@ -123,6 +130,7 @@ async def select_due_sources(session: AsyncSession) -> list[Source]:
         .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()
 
     cooldowns = await _platforms_in_cooldown(session)