feat: implement Phase 4 enhancements

- Migrate frontend to Alpine.js for reactive state management
- Add source health dashboard in UI and /api/source-health endpoint
- Add event tagging (PATCH /api/events/{id}/tags) and commenting (POST /api/events/{id}/comments)
- Add CSV/JSON export from the UI
- Add rule-based alerting engine (rules.py) with CRUD endpoints (/api/rules)
- Add SIEM export via webhook (siem.py)
- Add AOC audit trail middleware logging all mutations to aoc_audit collection
- Update config with SIEM_ENABLED, SIEM_WEBHOOK_URL, ALERTS_ENABLED
- Add tests for rules engine, tags, comments, and source health

backend/audit_trail.py

@@ -0,0 +1,22 @@
+from datetime import UTC, datetime
+
+import structlog
+from database import db
+
+logger = structlog.get_logger("aoc.audit")
+audit_collection = db["aoc_audit"]
+
+
+def log_action(action: str, resource: str, details: dict | None = None, user: str | None = None):
+    """Log an action in the AOC audit trail."""
+    doc = {
+        "timestamp": datetime.now(UTC).isoformat(),
+        "action": action,
+        "resource": resource,
+        "details": details or {},
+        "user": user or "anonymous",
+    }
+    try:
+        audit_collection.insert_one(doc)
+    except Exception as exc:
+        logger.warning("Failed to write audit trail", error=str(exc))