Router: validate Deps.Regions when vehicle or weather routes are wired

[aaronbrethorst]

From CodeRabbit review on PR #1 (#1 (review)) — Stability, Trivial.

Both the vehicles and weather handlers call resolveRegion, which dereferences deps.Regions on every request. The loud-failure startup block in internal/httpapi/router.go (~148-161) only runs when deps.Auth != nil, so a configuration that sets Deps.Vehicles or Deps.Weather with a nil Deps.Regions passes startup and nil-dereferences on the first request.

buildDeps always sets Regions today, so this is a validation gap rather than a live defect. Extend the startup check to panic when Deps.Regions is nil while either service is wired.