feat: Add update_project_user_permission method to SDK

- Add new method to update or assign user permissions for projects
- Support roles: annotator, reviewer, owner, and none (for removal)
- Update README with usage examples

This enables SDK users to manage project user permissions programmatically,
matching the API endpoint added in fastlabel-application PR #10438.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: rikunosuke

README.md

@@ -2547,6 +2547,32 @@ project_metadata_id = client.update_project_metadata(
 )
 ```
 
+### Update Project User Permission
+
+Update or assign user permissions for a project.
+
+```python
+# Add or update user permission
+result = client.update_project_user_permission(
+  project="YOUR_PROJECT_SLUG",
+  email="[email protected]",
+  role="annotator"
+)
+
+# Remove user from project
+result = client.update_project_user_permission(
+  project="YOUR_PROJECT_SLUG",
+  email="[email protected]",
+  role="none"
+)
+```
+
+Available roles:
+- `annotator`: Annotator
+- `reviewer`: Reviewer  
+- `owner`: Project owner
+- `none`: Remove user from project
+
 ## Tags
 
 ### Get Tags

fastlabel/__init__.py

@@ -4137,6 +4137,30 @@ def update_project_metadata(
         }
         return self.api.put_request(endpoint, payload=payload)
 
+    def update_project_user_permission(
+        self,
+        project: str,
+        email: str,
+        role: str,
+    ) -> bool:
+        """
+        Update project user permission.
+
+        project is the slug of the project (Required).
+        email is the email address of the user (Required).
+        role is the role of the user in the project (Required).
+        role can be 'annotator', 'reviewer', 'owner', 
+        'external_annotator', 'external_reviewer', 'external_approver', 
+        'external_sub_owner', 'external_owner', or 'none' to remove user from project.
+        """
+        endpoint = "projects-users"
+        payload = {
+            "project": project,
+            "email": email,
+            "role": role
+        }
+        return self.api.put_request(endpoint, payload=payload)
+
     # Tags
 
     def get_tags(