feat: include measurement_start_time in cached /status response

So consumers (e.g. the command center) can read the elapsed-time clock from
the cached status instead of a fresh device /live read. Added to both the
GET and POST /status data dicts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/routers.py

@@ -785,6 +785,7 @@ def get_status(unit_id: str, db: Session = Depends(get_db)):
             "unit_id": unit_id,
             "last_seen": status.last_seen.isoformat() if status.last_seen else None,
             "measurement_state": status.measurement_state,
+            "measurement_start_time": status.measurement_start_time.isoformat() if status.measurement_start_time else None,
             "lp": status.lp,
             "leq": status.leq,
             "lmax": status.lmax,
@@ -843,6 +844,7 @@ def upsert_status(unit_id: str, payload: StatusPayload, db: Session = Depends(ge
             "unit_id": unit_id,
             "last_seen": status.last_seen.isoformat(),
             "measurement_state": status.measurement_state,
+            "measurement_start_time": status.measurement_start_time.isoformat() if status.measurement_start_time else None,
             "lp": status.lp,
             "leq": status.leq,
             "lmax": status.lmax,