Backend missing a /health endpoint for basic service status checks

### Problem
Currently, the backend does not expose a simple `/health` (or similar) endpoint to verify whether the server is running correctly.

During local development, contributors have no easy way to distinguish between:
- backend not running
- backend running but DB connection failed
- backend running but routes not reachable

This caused confusion when trying to access the frontend, which resulted in "localhost refused to connect" errors without a clear backend signal.

### Expected Behavior
The backend should expose a lightweight health check endpoint (e.g. `GET /health`) that:
- returns HTTP 200 when the server is running
- returns a small JSON response such as `{ "status": "ok" }`

This endpoint should not require authentication and should not depend on optional services like Gemini or Redis.

### Actual Behavior
There is no simple endpoint to verify backend availability, making debugging and onboarding harder for new contributors.

### Impact
- Harder local setup and debugging
- Poor contributor experience
- No simple way to verify backend status from browser or scripts

### Suggested Fix
Add a basic health check route that:
- returns a static JSON response
- does not perform heavy checks
- is safe for local development and CI usage

This would significantly improve developer experience.

Could you please assign this issue to me so I can work on it?