Set up Ktor integration tests with test engine

[GavT]

Summary

There are no integration tests for the Ktor routes. Ktor provides a testApplication engine that runs the full stack in-memory without starting a real HTTP server.

What to add

1. Add ktor-server-test-host to testImplementation dependencies
2. Create RoutesTest.kt:
   • GET /api/health returns 200 when cache is ready
   • GET /api/search?postcode=SW1A1AA&radius=5 returns station list
   • GET /api/search without postcode returns 400
   • GET /api/search?postcode=INVALID returns appropriate error
3. Use MockDataCache that bypasses UK Gov API calls

Workstream: WS2 Phase 2B