feat: add marketplace metrics, privacy features, and service registry endpoints

- Add Prometheus metrics for marketplace API throughput and error rates with new dashboard panels
- Implement confidential transaction models with encryption support and access control
- Add key management system with registration, rotation, and audit logging
- Create services and registry routers for service discovery and management
- Integrate ZK proof generation for privacy-preserving receipts
- Add metrics instru

apps/coordinator-api/src/app/metrics.py

@@ -0,0 +1,16 @@
+"""Prometheus metrics for the AITBC Coordinator API."""
+
+from prometheus_client import Counter
+
+# Marketplace API metrics
+marketplace_requests_total = Counter(
+    'marketplace_requests_total',
+    'Total number of marketplace API requests',
+    ['endpoint', 'method']
+)
+
+marketplace_errors_total = Counter(
+    'marketplace_errors_total',
+    'Total number of marketplace API errors',
+    ['endpoint', 'method', 'error_type']
+)