fix(test): drop stale 'starts at 0' assertion after auto-arm-on-create

`test_set_backfill_runs_arms_source` was pinning the pre-auto-arm initial
value (0) when checking that the override works. Now that create() pre-arms
enabled sources to 3, the assertion is stale — the test was already
verifying the override path, the pre-assertion just decorated it.

Drop the pre-assertion; keep the override check. Other tests use raw
Source(...) constructors (default to 0 via the column server_default),
not SourceService.create(), so they're unaffected.

tests/test_source_service.py

@@ -176,15 +176,15 @@ async def test_list_hides_sidecar_synthetic_anchors(db):
 
 @pytest.mark.asyncio
 async def test_set_backfill_runs_arms_source(db):
-    """The service method sets backfill_runs_remaining and returns the
-    updated record so the API can echo it back."""
+    """The service method overrides backfill_runs_remaining (regardless of
+    the auto-arm-on-create starting value) and returns the updated record
+    so the API can echo it back."""
     artist = await _artist(db, "Alice")
     svc = SourceService(db)
     rec = await svc.create(
         artist_id=artist.id, platform="patreon",
         url="https://patreon.com/alice",
     )
-    assert rec.backfill_runs_remaining == 0
     updated = await svc.set_backfill_runs(rec.id, 5)
     assert updated.backfill_runs_remaining == 5