fix: say when the camera catalog serves only part of a region

Camera packs keep their nearest cameras and the catalog cap thins them
further, so London answered 250 cameras while TfL offers many more. The
catalog now records each pack's full count, tags cameras with their pack,
and serves the packs it trimmed; find_cctv_cameras names them.

server/providers/cctv/normalize.js

@@ -470,7 +470,10 @@ export function prioritizeSources(cameras, maxCount, anchors) {
     return a.idx - b.idx;
   });
 
-  return scored.slice(0, cap).map((entry) => entry.camera);
+  const kept = scored.slice(0, cap).map((entry) => entry.camera);
+  // How many cameras the pack offered before trimming, for coverage reports.
+  Object.defineProperty(kept, 'available', { value: list.length });
+  return kept;
 }
 
 /**