Age a stalled transfer from created_at when started_at is missing

The stall clock read started_at and returned zero when it was absent, which
exempted every row written before that column existed -- precisely the jobs
that have been stuck the longest. The live instance had one: a `downloading`
job from before the upgrade, its transfer parked in Queued, Remotely, which
the new check would have skipped forever.

Falls back to created_at, which for a job still in `downloading` is the same
question asked a slightly different way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app.py

@@ -1261,8 +1261,16 @@ def transfer_index(payload: list) -> dict[str, dict]:
 
 
 def stalled_for(job: sqlite3.Row) -> float:
-    """Minutes since this transfer last had anything to show for itself."""
-    started = job["started_at"] if "started_at" in job.keys() else None
+    """Minutes since this transfer last had anything to show for itself.
+
+    Falls back to created_at, which is what makes this work on rows written
+    before started_at existed. Those are `downloading` with no clock at all,
+    so returning zero for them would exempt precisely the jobs that have been
+    stuck the longest -- the ones this check exists to catch.
+    """
+    keys = job.keys()
+    started = ((job["started_at"] if "started_at" in keys else None)
+               or (job["created_at"] if "created_at" in keys else None))
     if not started:
         return 0.0
     try: