[FEATURE] Build Validate Milestone Completion Endpoint

**Describe the solution you'd like**
Build an endpoint to allow SMEUsers to upload documents to show completion of each task on the Project Details list.

**IMPORTANT**
Write tests.

No files, entry points, or existing endpoints are named. Start by locating the Python backend's route definitions and existing SMEUser, project-task, and document-upload handling. Done means an endpoint accepts documents for each task's completion, with the expected validation behavior covered by tests.

Written by the indexing model from the issue text.