feat: Complete assets proxy implementation with documentation

- Add comprehensive documentation in docs/ASSETS_PROXY.md
- Add contract comments in normalizeAssetUrl and proxy_asset
- Verify all components use normalizeAssetUrl
- Verify ENV variables are correctly set
- Add troubleshooting guide

services/city-service/routes_city.py

@@ -328,7 +328,22 @@ async def update_agent_visibility_endpoint(
 async def proxy_asset(path: str):
     """
     Proxy endpoint for serving MinIO assets.
-    Allows serving assets through /api/assets/... instead of requiring assets.daarion.space DNS.
+    
+    Allows serving assets through /api/city/assets/proxy/... instead of requiring 
+    assets.daarion.space DNS setup.
+    
+    CONTRACT with normalizeAssetUrl:
+    - Input from normalizeAssetUrl: /api/city/assets/proxy/microdao/logo/2025/12/02/abc123.png
+    - Path parameter (after /city/assets/proxy/): microdao/logo/2025/12/02/abc123.png
+    - MinIO object key: daarion-assets/microdao/logo/2025/12/02/abc123.png
+      (ASSETS_BUCKET is prepended automatically)
+    
+    Args:
+        path: Object path in MinIO (without bucket prefix)
+              Example: "microdao/logo/2025/12/02/abc123.png"
+    
+    Returns:
+        StreamingResponse with image data and appropriate Content-Type
     """
     from lib.assets_client import get_minio_client, ASSETS_BUCKET
     from fastapi.responses import StreamingResponse
@@ -338,6 +353,8 @@ async def proxy_asset(path: str):
         client = get_minio_client()
         
         # Get object from MinIO
+        # path already contains the full object key (e.g., "microdao/logo/2025/12/02/abc123.png")
+        # ASSETS_BUCKET is prepended by get_object()
         response = client.get_object(ASSETS_BUCKET, path)
         
         # Read data