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/api/v1/events.py

@@ -1,7 +1,9 @@
+import logging
 from datetime import datetime
 from typing import Any
 
 from fastapi import APIRouter, Body, Depends, HTTPException, Query
+from fastapi.responses import JSONResponse
 from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session, joinedload
@@ -18,6 +20,7 @@ from app.services.schema_validate import validate_event_payload
 from app.services.telegram_notify import notify_event, notify_problem
 
 router = APIRouter(prefix="/events", tags=["events"])
+logger = logging.getLogger("sac.ingest")
 
 
 class IngestResponse(BaseModel):
@@ -28,14 +31,32 @@ class IngestResponse(BaseModel):
     problem_id: int | None = None
 
 
-@router.post("", status_code=202, response_model=IngestResponse)
+def _ingest_response(event: Event, *, created: bool) -> IngestResponse:
+    settings = get_settings()
+    base = settings.sac_public_url.rstrip("/")
+    return IngestResponse(
+        status="created" if created else "duplicate",
+        event_id=event.event_id,
+        created=created,
+        sac_event_url=f"{base}/api/v1/events/{event.id}",
+        problem_id=None,
+    )
+
+
+@router.post("", response_model=IngestResponse)
 def post_event(
     payload: dict[str, Any] = Body(...),
     db: Session = Depends(get_db),
     _api_key: str = Depends(get_api_key_auth),
-) -> IngestResponse:
+) -> JSONResponse:
     errors = validate_event_payload(payload)
     if errors:
+        event_id_hint = payload.get("event_id") if isinstance(payload.get("event_id"), str) else None
+        logger.warning(
+            "ingest rejected status=422 event_id=%s errors=%s",
+            event_id_hint,
+            errors[:5],
+        )
         raise HTTPException(status_code=422, detail={"schema_errors": errors[:20]})
 
     event, created = ingest_event(db, payload)
@@ -46,17 +67,16 @@ def post_event(
         notify_event(event)
         if problem is not None and problem_created:
             notify_problem(problem, event)
+        logger.info("ingest created event_id=%s type=%s host_id=%s", event.event_id, event.type, event.host_id)
+    else:
+        logger.info("ingest duplicate event_id=%s", event.event_id)
     db.commit()
 
-    settings = get_settings()
-    base = settings.sac_public_url.rstrip("/")
-    return IngestResponse(
-        status="accepted",
-        event_id=event.event_id,
-        created=created,
-        sac_event_url=f"{base}/api/v1/events/{event.id}",
-        problem_id=problem.id if problem else None,
-    )
+    body = _ingest_response(event, created=created)
+    if problem is not None:
+        body.problem_id = problem.id
+    status_code = 201 if created else 409
+    return JSONResponse(status_code=status_code, content=body.model_dump(mode="json"))
 
 
 def _parse_optional_dt(value: str | None) -> datetime | None: