Refresh docs and client for backend-routed asset uploads.

This updates developer docs and web demos to use backend upload endpoints, adds a client upload helper, and aligns integration tests with the no-direct-MinIO URL flow.

Made-with: Cursor

docs/docker-minio-local-dev.md

@@ -40,8 +40,8 @@ docker compose up --build -d
    - `http://localhost:8774`
 3. Confirm bucket exists (`momswap-assets` by default).
 4. Use API flow:
-   - create asset and get signed upload URL
-   - upload file with PUT
+   - create asset and request backend upload URL
+   - upload file with `PUT /v1/assets/{id}/upload`
    - request `/v1/assets/{id}/download`
 
 ## Quick verification script
@@ -67,6 +67,6 @@ fi
 - If bucket bootstrap fails, inspect:
   - `docker compose logs minio`
   - `docker compose logs minio-init`
-- If signed URLs are generated but upload fails, check:
+- If backend upload endpoint fails, check:
   - object key path style (`S3_USE_PATH_STYLE=true` for MinIO)
   - MinIO credentials (`S3_ACCESS_KEY`, `S3_SECRET_KEY`)