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>

tests/test_scheduler_service.py

@@ -233,6 +233,45 @@ async def test_select_ignores_expired_cooldown(db):
     assert any(s.url == "https://cd-exp" for s in due)
 
 
+@pytest.mark.asyncio
+async def test_select_orders_most_overdue_first_then_id(db):
+    """Within the due set, sources are ordered by last_checked_at ASC NULLS
+    FIRST. Combined with Celery FIFO on the download queue, the most-overdue
+    source in each tick reaches a worker first — preventing the 'a freshly-
+    rerun source keeps cutting in line ahead of one that's still waiting'
+    starvation pattern when queue throughput is below the tick population."""
+    artist = await _seed_artist(db, interval=60, name="order-test")
+    now = datetime.now(UTC)
+    # Newest check — least overdue (but still past its 60s interval).
+    s_new = Source(
+        artist_id=artist.id, platform="patreon", url="https://order-new",
+        enabled=True, consecutive_failures=0,
+        last_checked_at=now - timedelta(minutes=2),
+    )
+    # Oldest check — most overdue among checked sources.
+    s_old = Source(
+        artist_id=artist.id, platform="patreon", url="https://order-old",
+        enabled=True, consecutive_failures=0,
+        last_checked_at=now - timedelta(hours=4),
+    )
+    # Never checked — wins the ordering (NULLS FIRST).
+    s_never = Source(
+        artist_id=artist.id, platform="patreon", url="https://order-never",
+        enabled=True, consecutive_failures=0,
+    )
+    db.add_all([s_new, s_old, s_never])
+    await db.commit()
+
+    due = await select_due_sources(db)
+    # Filter to just our test seeds (concurrent tests may add others).
+    urls = [s.url for s in due if s.url.startswith("https://order-")]
+    assert urls == [
+        "https://order-never",
+        "https://order-old",
+        "https://order-new",
+    ]
+
+
 @pytest.mark.asyncio
 async def test_set_platform_cooldown_upserts(db):
     """Calling set_platform_cooldown twice on the same platform updates