🐛(backend) fix invitations API endpoint access rights

Only users who have the rights to manage accesses on the document should
be allowed to see and manipulate invitations. Other users can see access
rights on the document but only when the corresponding user/team has
actually been granted access.

We added a parameter in document abilities so the frontend knows when
the logged-in user can invite another user with the owner role or not.

src/backend/core/api/permissions.py

@@ -1,9 +1,12 @@
 """Permission handlers for the impress core app."""
 
 from django.core import exceptions
+from django.db.models import Q
 
 from rest_framework import permissions
 
+from core.models import DocumentAccess, RoleChoices
+
 ACTION_FOR_METHOD_TO_PERMISSION = {
     "versions_detail": {"DELETE": "versions_destroy", "GET": "versions_retrieve"}
 }
@@ -59,6 +62,38 @@ class IsOwnedOrPublic(IsAuthenticated):
             return False
 
 
+class CanCreateInvitationPermission(permissions.BasePermission):
+    """
+    Custom permission class to handle permission checks for managing invitations.
+    """
+
+    def has_permission(self, request, view):
+        user = request.user
+
+        # Ensure the user is authenticated
+        if not (bool(request.auth) or request.user.is_authenticated):
+            return False
+
+        # Apply permission checks only for creation (POST requests)
+        if view.action != "create":
+            return True
+
+        # Check if resource_id is passed in the context
+        try:
+            document_id = view.kwargs["resource_id"]
+        except KeyError as exc:
+            raise exceptions.ValidationError(
+                "You must set a document ID in kwargs to manage document invitations."
+            ) from exc
+
+        # Check if the user has access to manage invitations (Owner/Admin roles)
+        return DocumentAccess.objects.filter(
+            Q(user=user) | Q(team__in=user.teams),
+            document=document_id,
+            role__in=[RoleChoices.OWNER, RoleChoices.ADMIN],
+        ).exists()
+
+
 class AccessPermission(permissions.BasePermission):
     """Permission class for access objects."""