fix: a never-checked source still reports no next check while its first backfill is armed

Every new source is created with a backfill armed, so e5bdcd2's backfill
branch in compute_next_check_at answered "now" for sources that have never
been checked, where the contract (test_create_response_includes_fc3d_fields)
is None. The never-checked case comes first again. Scheduling is unaffected:
select_due_sources already treats never-checked sources as due.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/services/scheduler_service.py

@@ -194,10 +194,10 @@ 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
+    if backfill_ready(source):
+        return datetime.now(UTC)
     interval = compute_effective_interval(source, artist, settings)
     return source.last_checked_at + timedelta(seconds=interval)