feat: Admin Operations SIEM — alerts, notifications, pre-built rules

- Add pluggable notification system (webhook, Slack, Teams) with retry
- Add alert deduplication: same rule + actor within 15 min = one alert
- Add 10 pre-built admin-ops rule templates seeded on startup:
  - Failed Conditional Access, After-Hours Admin Activity
  - New Application Registration, Admin Role Assignment
  - License Change, Bulk User Deletion
  - Device Compliance Failure, Exchange Transport Rule Change
  - Service Principal Credential Added, External Sharing Enabled
- Add /api/alerts, /api/alerts/{id}/status, /api/alerts/summary endpoints
- Add alert dashboard to frontend with status filters and ack/resolve buttons
- Add alert summary badge in hero header (high/medium/low counts)
- New env vars: ALERT_WEBHOOK_URL, ALERT_WEBHOOK_FORMAT, ALERT_DEDUPE_MINUTES

backend/tests/test_rules.py

@@ -59,6 +59,7 @@ def test_evaluate_event_creates_alert(monkeypatch):
         inserted["doc"] = doc
 
     monkeypatch.setattr(alerts_collection, "insert_one", mock_insert)
+    monkeypatch.setattr(alerts_collection, "count_documents", lambda *args, **kwargs: 0)
 
     event = {"id": "e1", "operation": "Add user", "timestamp": datetime.now(UTC).isoformat(), "dedupe_key": "dk1"}
     triggered = evaluate_event(event)