feat: add Docker health checks to all microservices

[Mrsandeep27]

Summary

Closes #1196

• Added healthcheck blocks to all services in docker-compose.yml
• Infrastructure: postgres (pg_isready), redis (redis-cli ping), pgbouncer
• Application: all 6 Spring Boot services via /actuator/health endpoint
• Gateway: nginx via curl
• Replaced bare depends_on with condition: service_healthy for proper startup ordering
• Tuned intervals: infra (10s/5s timeout), apps (30s/10s timeout, 60s start_period)

Test plan

• Run docker compose up and verify services start in correct order
• Verify docker compose ps shows health status for all services
• Confirm unhealthy service prevents dependent services from starting

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