Add test coverage for zone and live sensor endpoints

[SL-Mar]

Problem

tests/integration/test_api.py covers vessel, route, and fuel endpoints but has zero tests for:

• Zone CRUD: POST /api/zones, DELETE /api/zones/{id}
• Zone queries: /api/zones/at-point, /api/zones/check-path
• Live sensor: /api/live/status, /api/live/connect, WebSocket /api/live/ws
• Calibration estimation: /api/vessel/calibration/estimate-fouling
• Data sources: GET /api/data-sources

Steps to fix

1. Add zone endpoint tests (CRUD, polygon intersection, GeoJSON export)
2. Add live sensor endpoint tests (status, connect/disconnect lifecycle)
3. Add calibration estimation and data source tests
4. Target >85% endpoint coverage

Files

• tests/integration/test_api.py