feat(projects): add projects app's basic structure and endpoints

apps/projects/services/memberships.py

@@ -0,0 +1,32 @@
+from rest_framework.exceptions import ValidationError
+from apps.projects.models import ProjectMembership
+
+def add_project_member(project, user, 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():
+        raise ValidationError({"user_id": "This user is already a member of the project."})
+
+    return ProjectMembership.objects.create(
+        project=project,
+        user=user,
+        role=role,
+        is_active=True
+    )
+
+def update_project_member(membership, **kwargs):
+    """
+    Updates a project membership (e.g., changing role or active status).
+    """
+    update_fields = []
+    for field, value in kwargs.items():
+        if hasattr(membership, field) and getattr(membership, field) != value:
+            setattr(membership, field, value)
+            update_fields.append(field)
+            
+    if update_fields:
+        update_fields.append("updated_at")
+        membership.save(update_fields=update_fields)
+        
+    return membership