Enhance /api/healthz with DB connectivity check

[xcompass]

Summary

• Replace static {'status': 'OK'} response with a real health check that verifies database connectivity
• Return 503 Service Unavailable (with error detail) when any check fails, so monitoring services can detect outages
• Add tests covering the healthy path, DB-down path, and unauthenticated access

Changes

compair/api/healthz.py

• Execute SELECT 1 against the database to confirm connectivity
• Response body now includes a checks map ({"db": "ok"|"error"}) for granular visibility
• HTTP status is 200 when all checks pass, 503 otherwise

compair/tests/api/test_healthz.py (new)

• test_healthz_returns_200_when_db_is_up — happy path
• test_healthz_returns_503_when_db_is_down — DB failure mocked via mock.patch
• test_healthz_requires_no_authentication — confirms endpoint is publicly reachable

Test plan

• Run docker exec -it compair_app_1 python -m pytest compair/tests/api/test_healthz.py -v
• Hit GET /api/healthz on a running instance and confirm 200 {"status": "ok", "checks": {"db": "ok"}}
• Confirm monitoring service receives 503 when the DB is unreachable

[catinhere]

LGTM and executed everything under the test plan