Use Proper HTTP Status Codes on DB Connection Failures

* **Gap**: API endpoints return HTTP 500 when database instances are unreachable, hiding service status.
* **Relevant Files**:
  - `src/codegraphcontext/api/router.py`
* **Task**:
  - Intercept database socket errors and raise an HTTP 503 Service Unavailable exception.
* **Acceptance Criteria**:
  - Service status is cleanly reported on backend drops.
