feat(leads): link consumer account to lead on submission

- user_account FK on Lead model (SET_NULL, related_name='submitted_leads')
- Migration 0004_lead_user_account
- ScheduleCallView auto-matches consumer account by email on create
- _serialize_lead now returns userAccount: {id, name, email, phone, eventifyId, profilePicture}

mobile_api/views/user.py

@@ -491,6 +491,14 @@ class ScheduleCallView(View):
             if errors:
                 return JsonResponse({'errors': errors}, status=400)
 
+            # Auto-link to a consumer account if one exists with this email
+            from django.contrib.auth import get_user_model
+            _User = get_user_model()
+            try:
+                consumer_account = _User.objects.get(email=email)
+            except _User.DoesNotExist:
+                consumer_account = None
+
             lead = Lead.objects.create(
                 name=name,
                 email=email,
@@ -500,6 +508,7 @@ class ScheduleCallView(View):
                 status='new',
                 source='schedule_call',
                 priority='medium',
+                user_account=consumer_account,
             )
             log("info", f"New schedule-call lead #{lead.pk} from {email}", request=request)
             return JsonResponse({