Extract common database functionality from api/ntpdb and monitor/ntpdb
into shared common/database package:
- Dynamic connector pattern with configuration loading
- Configurable connection pool management (API: 25/10, Monitor: 10/5)
- Optional Prometheus metrics integration
- Generic transaction helpers with proper error handling
- Unified interfaces compatible with SQLC-generated code
Foundation for migration to eliminate ~200 lines of duplicate code.
