Fix N+1 query issue in Grant admin invitation letter flag

Use Django''s Exists annotation in get_queryset() instead of calling
has_invitation_letter_request() method for each grant in the admin list.
This optimizes the query to use a single SQL query with EXISTS subquery
rather than individual queries for each grant row.

Co-authored-by: Marco Acierno <marcoacierno@users.noreply.github.com>

Author: claude[bot]

backend/grants/admin.py

@@ -588,9 +588,11 @@ def has_voucher(self, obj: Grant) -> bool:
     @admin.display(description="📧", boolean=True)
     def has_invitation_letter_request_flag(self, obj: Grant) -> bool:
         """Display flag indicating if user has submitted an invitation letter request"""
-        return obj.has_invitation_letter_request()
+        return getattr(obj, 'has_invitation_letter_request', False)
 
     def get_queryset(self, request):
+        from visa.models import InvitationLetterRequest
+        
         qs = (
             super()
             .get_queryset(request)
@@ -614,6 +616,12 @@ def get_queryset(self, request):
                         user_id=OuterRef("user_id"),
                     )
                 ),
+                has_invitation_letter_request=Exists(
+                    InvitationLetterRequest.objects.filter(
+                        conference_id=OuterRef("conference_id"),
+                        requester_id=OuterRef("user_id"),
+                    )
+                ),
             )
         )
 

backend/grants/models.py

@@ -333,16 +333,6 @@ def has_approved_accommodation(self):
     def current_or_pending_status(self):
         return self.pending_status or self.status
 
-    def has_invitation_letter_request(self):
-        """Check if user has submitted an invitation letter request for this conference"""
-        if not self.user_id:
-            return False
-        
-        from visa.models import InvitationLetterRequest
-        return InvitationLetterRequest.objects.filter(
-            conference=self.conference,
-            requester=self.user
-        ).exists()
 
 
 class GrantConfirmPendingStatusProxy(Grant):