fix(cron): make naive ISO timestamps timezone-aware at parse time

User-provided ISO timestamps like '2026-02-03T14:00' (no timezone)
were stored naive. The _ensure_aware() helper at check time interprets
naive datetimes using the current system timezone, but if the system
timezone changes between job creation and checking, the job fires at
the wrong time.

Fix: call dt.astimezone() at parse time to immediately stamp the
datetime with the local timezone. The stored value is now always
timezone-aware, so it's stable regardless of later timezone changes.

cron/jobs.py

@@ -168,6 +168,10 @@ def parse_schedule(schedule: str) -> Dict[str, Any]:
         try:
             # Parse and validate
             dt = datetime.fromisoformat(schedule.replace('Z', '+00:00'))
+            # Make naive timestamps timezone-aware at parse time so the stored
+            # value doesn't depend on the system timezone matching at check time.
+            if dt.tzinfo is None:
+                dt = dt.astimezone()  # Interpret as local timezone
             return {
                 "kind": "once",
                 "run_at": dt.isoformat(),