Add MapLibre demo and route uploads through backend.

This introduces a MapLibre GL + Three.js web demo for object placement and sharing, and changes asset upload flow to use backend upload endpoints so clients no longer receive direct MinIO URLs.

Made-with: Cursor

docs/frontend-development.md

@@ -32,6 +32,7 @@ web/
 2. Open:
    - `http://localhost:8122/web/`
    - `http://localhost:8122/web/leaflet-demo.html` (Leaflet map demo for 3D/image placement + sharing)
+   - `http://localhost:8122/web/maplibre-demo.html` (MapLibre GL vector tiles + Three.js object rendering/placement)
 
 ### Runtime dependencies
 
@@ -60,6 +61,11 @@ web/
   - click map to place object coordinates
   - create feature + upload/link `gltf`/`glb`/image asset
   - copy/open share link and toggle public/private visibility
+- MapLibre GL + Three.js example:
+  - vector tile basemap via MapLibre style
+  - map click to place object position
+  - 3D marker rendering in custom Three.js layer
+  - asset upload/link and share/visibility controls backed by API
 
 ## TypeScript client (`libs/geo-api-client`)