feat: add /health check and modernize public endpoints

[ayushshukla1807]

This PR introduces several production-grade enhancements to the public API layer.

Changes:

• Health Check Endpoint: Added GET /health to verify database connectivity. This is essential for modern container orchestration and uptime monitoring.
• Security Hardening: Removed insecure hardcoded debug impersonation logic. Replaced with a structured allow_impersonation config flag that only functions in the dev environment.
• Modernization: Added comprehensive type hints to public_endpoints.py and updated path handling using os.path.join for better cross-platform compatibility.
• Refactoring: Cleaned up legacy TODO comments and unused logic in the login flow.

These changes significantly improve the security posture and maintainability of the public entry points.

[lgelauff]

According to the docs, /meta seems to fulfill mostly the same functionality. Could you verify for duplication?

[ayushshukla1807]

I've updated /health to return a 503 Service Unavailable on DB failure. While /meta provides app internals, this endpoint serves as a dedicated, public-facing probe for load balancers and external monitoring. All commits are now DCO-signed as well.

[ayushshukla1807]

I've updated /health to return a 503 Service Unavailable on DB failure. While /meta provides app internals, this endpoint serves as a dedicated, public-facing probe for load balancers and external monitoring. All commits are now DCO-signed as well.