fix(jira): store execution times as real UTC, fix RT date fields, label reopens

- execution_start_time/execution_end_time/detection_time/containment_time
  were captured from a datetime-local input with no local->UTC conversion,
  then stored/displayed as if already UTC — off by the browser's offset.
  Frontend now converts properly in both directions; backend schemas
  defensively normalize any tz-aware input to naive UTC as well.
- push_rt_submitted was pushing datetime.utcnow() (whenever the submit
  button was clicked) into Jira's RT Start/End Date fields instead of the
  operator-entered execution window. Now pushes the *first* round's
  execution start (recovered from round_history across reopens) and the
  *current* round's execution end. Removed push_rt_started, which only
  ever pushed a meaningless click-timestamp.
- Reopening a test now adds a "reopened" Jira label (read-modify-write so
  existing labels aren't clobbered), on top of the existing round-archived
  comment and status push.

backend/app/services/test_workflow_service.py

@@ -243,12 +243,6 @@ def start_execution(db: Session, test: Test, user: User) -> Test:
     except Exception as e:
         logger.warning("Jira push failed for test %s: %s", test.id, e, exc_info=True)
 
-    try:
-        from app.services.jira_service import push_rt_started
-        push_rt_started(db, test)
-    except Exception as e:
-        logger.warning("Jira RT start date push failed for test %s: %s", test.id, e, exc_info=True)
-
     return test