refactor: production-grade containerization and deployment architecture

[iamjaygao]

Deployment & Infrastructure Refactor

Refactored the system to production-grade containerization with a focus on reliability, security, and scalability.

• removed database migration from container startup to eliminate race conditions across multiple containers
• introduced non-root user to follow least-privilege security practices
• implemented multi-stage builds to reduce image size and improve deployment efficiency
• improved gunicorn configuration with environment-driven workers and timeout to handle long-running AI requests
• optimized docker-compose by reusing backend image across services, reducing redundant builds
• enhanced nginx with security headers and rate limiting to mitigate abuse and improve system robustness
• removed unused frontend Dockerfile to prevent misconfiguration and simplify deployment