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/store_dwell_alert/app/manage_api.py

@@ -212,23 +212,25 @@ def _build_summary(ctx: ManageContext) -> dict:
                 continue
 
             if payload.get("event") == "half_hour_report":
-                last_report_time = _string_value(payload.get("window_end"))
-                active_count = _int_value(payload.get("active_customer_count"))
                 stat = _build_window_stat(payload)
                 window_stats.append(stat)
                 longest_dwell_seconds = max(
                     longest_dwell_seconds,
                     stat["max_wait_seconds"],
                 )
-                queue_level = stat["queue_level"]
-                over_threshold_count = stat["over_threshold_count"]
-                under_threshold_count = stat["under_threshold_count"]
-                status_change = stat["status_change"]
 
-    window_stats.sort(
-        key=lambda item: _parse_timestamp(item["window_end"]),
-        reverse=True,
-    )
+    window_stats.sort(key=lambda item: _sort_timestamp(item["window_end"]), reverse=True)
+
+    for stat in window_stats:
+        if _parse_timestamp(stat["window_end"]) is None:
+            continue
+        last_report_time = stat["window_end"]
+        active_count = stat["active_customer_count"]
+        queue_level = stat["queue_level"]
+        over_threshold_count = stat["over_threshold_count"]
+        under_threshold_count = stat["under_threshold_count"]
+        status_change = stat["status_change"]
+        break
 
     headline = "No reports yet"
     if last_report_time:
@@ -411,8 +413,20 @@ def _latest_timestamp(*values: str) -> str:
     return latest_raw
 
 
-def _parse_timestamp(value: str) -> datetime:
-    parsed = datetime.fromisoformat(value)
+def _sort_timestamp(value: str) -> tuple[int, datetime]:
+    parsed = _parse_timestamp(value)
+    if parsed is None:
+        return (0, datetime.min.replace(tzinfo=datetime.now().astimezone().tzinfo))
+    return (1, parsed)
+
+
+def _parse_timestamp(value: str) -> datetime | None:
+    if not value.strip():
+        return None
+    try:
+        parsed = datetime.fromisoformat(value)
+    except ValueError:
+        return None
     if parsed.tzinfo is None:
         return parsed.replace(tzinfo=datetime.now().astimezone().tzinfo)
     return parsed