feat: enrich member/comment/propose APIs with usernames

- ProjectMemberResponse now includes username and full_name
- Comment list endpoint returns author_username
- ProposeResponse now includes created_by_username
- All serializers resolve User objects to surface human-readable names
- Supports frontend code-first migration (TODO §3.1/3.2)

app/api/routers/projects.py

@@ -332,9 +332,12 @@ def list_project_members(project_id: str, db: Session = Depends(get_db)):
             role = db.query(Role).filter(Role.id == m.role_id).first()
             if role:
                 role_name = role.name
+        user = db.query(models.User).filter(models.User.id == m.user_id).first()
         result.append({
             "id": m.id,
             "user_id": m.user_id,
+            "username": user.username if user else None,
+            "full_name": user.full_name if user else None,
             "project_id": m.project_id,
             "role": role_name
         })