refactor: migrate to rbx-upload library

- Replace hand-rolled roblox_service.py and models.py with rbx-upload
- Initialize RobloxClient in FastAPI lifespan context manager
- Remove leaked singleton httpx.AsyncClient
- Simplify upload_clothing_image call (group_id int instead of RbxCreator)
- Fix GET /asset/{id} returning a set literal instead of a dict
- Drop httpx direct dependency (now transitive via rbx-upload)

pyproject.toml

@@ -7,7 +7,5 @@ requires-python = ">=3.13"
 dependencies = [
     "fastapi[standard]>=0.126.0",
     "python-dotenv>=1.2.1",
-    "httpx>=0.25.0",
+    "rbx-upload",
 ]
-
-