feat(projects): support members and align rate payloads

apps/projects/services/memberships.py

@@ -1,16 +1,16 @@
 from rest_framework.exceptions import ValidationError
 from apps.projects.models import ProjectMembership
 
-def add_project_member(project, user, role):
+def add_project_member(project, user_id, role):
     """
     Adds a user to a project. Ensures no duplicate active memberships exist.
     """
-    if ProjectMembership.objects.filter(project=project, user=user, is_deleted=False).exists():
+    if ProjectMembership.objects.filter(project=project, user_id=user_id, is_deleted=False).exists():
         raise ValidationError({"user_id": "This user is already a member of the project."})
 
     return ProjectMembership.objects.create(
         project=project,
-        user=user,
+        user_id=user_id,
         role=role,
         is_active=True
     )