feat(core): add robust health check endpoint with modular service checks

[mangopep]

Adds a robust, modular health check endpoint to the core app. The endpoint checks database and Redis connectivity (required), and OpenAI API (optional, only if key is set). Returns a JSON status for each service and an overall health indicator.

• Implements HealthCheckView in core/views.py using a registry-based approach for extensibility.
• Adds /core/health/ route in core/urls.py.
• DB and Redis are required for "healthy" status; OpenAI is optional.
• Returns HTTP 200 if core services are healthy, with clear JSON output.
• includes comprehensive tests for endpoint and error handling.

Closes #24

[DimitrovK]

Looks good!