Build the geospatial mapping service

[KingAkeem]

Summary

• replaces the retired companion backend with a FastAPI geospatial service
• adds dataset and X provider adapters with evidence-only location handling
• moves the standalone client to Vite, React 19, and React Leaflet 5
• adds cache, health, readiness, Docker, API documentation, and mocked tests

Verification

• 13 API tests pass
• 2 client tests pass
• production client build passes
• production dependency audit reports no known vulnerabilities

Release note

This is the service-side foundation for the coordinated platform and whoisit integration. The v1.0.0 release will follow after those consumers are merged and tested.