feat: ingest HTTP 201/409 for event_id idempotency

- Validate UUID format; log created/duplicate/rejected

- Tests for 201, 409, 422; update agent-integration and work-plan

- Docs: neutral IDE wording (no product-specific editor names)

backend/app/services/ingest.py

@@ -1,6 +1,7 @@
 from datetime import datetime, timezone
 
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app.models import Event, Host
@@ -85,5 +86,12 @@ def ingest_event(db: Session, payload: dict) -> tuple[Event, bool]:
         payload=payload,
     )
     db.add(event)
-    db.flush()
+    try:
+        db.flush()
+    except IntegrityError:
+        db.rollback()
+        raced = db.scalar(select(Event).where(Event.event_id == event_id))
+        if raced is not None:
+            return raced, False
+        raise
     return event, True