Retrieve domain invitations on first login

src/registrar/models/user.py

@@ -1,6 +1,8 @@
 from django.contrib.auth.models import AbstractUser
 from django.db import models
 
+from .domain_invitation import DomainInvitation
+
 from phonenumber_field.modelfields import PhoneNumberField  # type: ignore
 
 
@@ -31,3 +33,14 @@ class User(AbstractUser):
             return self.email
         else:
             return self.username
+
+    def first_login(self):
+        """Callback when the user is authenticated for the very first time.
+
+        When a user first arrives on the site, we need to retrieve any domain
+        invitations that match their email address.
+        """
+        for invitation in DomainInvitation.objects.filter(
+            email=self.email, status=DomainInvitation.SENT
+        ):
+            invitation.retrieve()