Switch the second Overpass pass (ways/relations) from out meta to out skel,
dropping unused tags/version/changeset fields from the response. The app only
reads structural references (node lists, relation members) from these elements.
Also inject http.Client into OverpassService for testability (matching
RoutingService pattern) and add close() for client lifecycle management.
14 tests covering query building, constraint detection, and error handling.
Fixes#108
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>