The /api/military-installations endpoint was the last keyless proxy without
the admitSameSite gate that protects terrain, CCTV, traffic, places, and
Celestrak. A cross-site <img>, foreign Origin, opaque origin, or
reverse-proxy-forwarded request now receives 403 before any Overpass query.
Same-origin app requests and bare tool requests continue to work.
Adds 4 integration tests (cross-site refused, same-origin admitted,
bare tool admitted, proxied refused) following the terrainHeightsBounds
test pattern.
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