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/core/stages/load_config.py

@@ -202,6 +202,16 @@ class LoadConfigStage(stage.BootingStage):
                 constants.instance_id = new_id
         constants.edition = ap.instance_config.data.get('system', {}).get('edition', 'community')
 
+        # Instance creation timestamp: sourced from data/labels/instance_id.json.
+        # Instances created before this field existed (or supplied via
+        # system.instance_id) won't have it, so backfill with the current time
+        # and persist it via the dump below — from then on it stays stable.
+        instance_create_ts = ap.instance_id.data.get('instance_create_ts', 0)
+        if not isinstance(instance_create_ts, int) or instance_create_ts <= 0:
+            instance_create_ts = int(time.time())
+            ap.instance_id.data['instance_create_ts'] = instance_create_ts
+        constants.instance_create_ts = instance_create_ts
+
         print(f'LangBot instance id: {constants.instance_id}')
         print(f'LangBot edition: {constants.edition}')
 

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(),

src/langbot/pkg/utils/constants.py

@@ -16,3 +16,11 @@ debug_mode = False
 edition = 'community'
 
 instance_id = ''
+
+instance_create_ts = 0
+"""Unix timestamp (seconds) of when this instance was first created.
+
+Sourced from ``data/labels/instance_id.json``. Backfilled to the current
+time for instances created before this field existed, so it is always a
+positive value once load_config has run.
+"""