feat: add default enrollment models

Adds two models, `DefaultEnterpriseEnrollmentIntention` and
`DefaultEnterpriseEnrollmentRealization`. The former defines, for a customer,
a course/run that associated users should be automatically enrolled into.
The latter represents the relationship between that intention record
and a realized EnterpriseCourseEnrollment; persisting realized enrollments
in this way will help to make read operations related to default enrollments
much more efficient.
ENT-9577

enterprise/models.py

@@ -2461,6 +2461,62 @@ class LicensedEnterpriseCourseEnrollment(EnterpriseFulfillmentSource):
     )
 
 
+class DefaultEnterpriseEnrollmentIntention(TimeStampedModel):
+    """
+    Specific to an enterprise customer, this model defines a course or course run
+    that should be auto-enrolled for any enterprise customer user linked to the customer.
+    """
+    uuid = models.UUIDField(
+        primary_key=True,
+        editable=False,
+    )
+    enterprise_customer = models.ForeignKey(
+        EnterpriseCustomer,
+        blank=False,
+        null=False,
+        related_name="default_course_enrollments",
+        on_delete=models.deletion.CASCADE,
+        help_text=_(
+            "The customer for which this default enrollment will be realized.",
+        )
+    )
+    content_key = models.CharField(
+        max_length=255,
+        blank=False,
+        null=False,
+        help_text=_(
+            "A course or course run that related users should be automatically enrolled into."
+        )
+    )
+    realized_enrollments = models.ManyToManyField(
+        EnterpriseCourseEnrollment,
+        through='RealizedDefaultEnterpriseEnrollment',
+        through_fields=("intended_enrollment", "realized_enrollment"),
+    )
+
+    @property
+    def current_course_run_key(self):  # pragma: no cover
+        """
+        The current run to use for realized course enrollments.
+        """
+        raise NotImplementedError
+
+
+class DefaultEnterpriseEnrollmentRealization(TimeStampedModel):
+    """
+    Represents the relationship between a `DefaultEnterpriseEnrollmentIntention`
+    and a realized course enrollment that exists because of that intention record.
+    """
+    intended_enrollment = models.ForeignKey(
+        DefaultEnterpriseEnrollmentIntention,
+        on_delete=models.CASCADE,
+    )
+    realized_enrollment = models.ForignKey(
+        EnterpriseCourseEnrollment,
+        on_delete=models.CASCADE,
+    )
+
+
 class EnterpriseCatalogQuery(TimeStampedModel):
     """
     Stores a re-usable catalog query.