feat(coordinator-api): add database connection pooling configuration and comprehensive payment validation
- Add configurable database connection pool settings (pool_size, max_overflow, pool_recycle, pool_pre_ping, echo)
- Replace hardcoded pool settings with environment-configurable values from settings
- Add QueuePool explicitly to both sync and async database engines
- Implement comprehensive Pydantic validators for payment schemas (JobPaymentCreate, PaymentRequest, ExchangePaymentRequest)
- Add regex