fix: configure PostgreSQL pool limits

[jamilahmadzai]

Summary

• configure TypeORM runtime and migration data sources with explicit PostgreSQL pool settings from DB_POOL_SIZE, capped at 20 connections per instance
• add idle/acquire timeouts, 1s slow-query warnings, and a warn-level pool saturation monitor for queued or fully busy pools
• document pool tuning and add a k6 /health/ready load test that exercises the database-backed readiness path at 2x the profile-read baseline

Validation

• npm ci
• npm test -- config/database-pool.config.spec.ts config/postgres-pool-monitor.spec.ts --runInBand --coverage --collectCoverageFrom=config/database-pool.config.ts --collectCoverageFrom=config/postgres-pool-monitor.ts (100% line coverage on changed config/monitor files)
• npm test -- --runInBand (115 tests)
• npm run build
• npm run lint
• node --check test/load/db-pool-readiness.js
• git diff --check

Closes #57

Bounty: GrantFox OSS campaign for issue #57