feat(telemetry): include instance_create_ts in heartbeat payload

Load the instance creation timestamp from data/labels/instance_id.json
(backfilling+persisting it for instances created before the field existed),
expose it as constants.instance_create_ts, and include it in the heartbeat
payload so Space can anchor Time-To-Value / onboarding analytics on real
install time rather than first-heartbeat.

Verified: py_compile, ruff, pytest tests/unit_tests/telemetry/ (37 passed).

src/langbot/pkg/telemetry/heartbeat.py

@@ -109,6 +109,7 @@ async def build_heartbeat_payload(ap: core_app.Application) -> dict:
         'query_id': '',
         'version': constants.semantic_version,
         'instance_id': constants.instance_id,
+        'instance_create_ts': constants.instance_create_ts,
         'edition': constants.edition,
         'features': features,
         'timestamp': datetime.now(timezone.utc).isoformat(),