feat: improve webhook filtering, worker status startup handling, and timestamp parsing

- Skip half_hour_report events from webhook posts in people_flow
- Handle pre-existing stale worker status files during startup gracefully
- Make store_dwell_alert timestamp parsing robust against invalid/empty values
- Update lessons learned and todo documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

managed/people_flow_project/src/people_flow/webhook.py

@@ -11,6 +11,10 @@ def _payload_for_webhook(payload: dict) -> dict:
     return outbound
 
 
+def _should_post_webhook(payload: dict) -> bool:
+    return payload.get("event") != "half_hour_report"
+
+
 def dispatch_json_event(
     path: str | Path,
     payload: dict,
@@ -22,7 +26,7 @@ def dispatch_json_event(
     with output_path.open("a", encoding="utf-8") as handle:
         handle.write(json.dumps(payload, ensure_ascii=False) + "\n")
 
-    if not webhook_url.strip():
+    if not webhook_url.strip() or not _should_post_webhook(payload):
         return
 
     req = request.Request(