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/database.py

@@ -8,6 +8,7 @@ client = MongoClient(MONGO_URI or "mongodb://localhost:27017")
 db = client[DB_NAME]
 events_collection = db["events"]
 saved_searches_collection = db["saved_searches"]
+alerts_collection = db["alerts"]
 logger = structlog.get_logger("aoc.database")