zilencer: Enforce uniqueness of server_id + remote_id.

This was previously just an index (not a unique one).  Enforce this
data constraint.

zilencer/migrations/0038_unique_server_remote_id.py

@@ -0,0 +1,34 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zilencer", "0037_alter_remoteinstallationcount_unique_together_and_more"),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="remoteinstallationcount",
+            constraint=models.UniqueConstraint(
+                condition=models.Q(("remote_id__isnull", False)),
+                fields=("server", "remote_id"),
+                name="unique_remote_installation_count_server_id_remote_id",
+            ),
+        ),
+        migrations.RemoveIndex(
+            model_name="remoteinstallationcount",
+            name="zilencer_remoteinstallat_server_id_remote_id_f72e4c30_idx",
+        ),
+        migrations.AddConstraint(
+            model_name="remoterealmcount",
+            constraint=models.UniqueConstraint(
+                condition=models.Q(("remote_id__isnull", False)),
+                fields=("server", "remote_id"),
+                name="unique_remote_realm_installation_count_server_id_remote_id",
+            ),
+        ),
+        migrations.RemoveIndex(
+            model_name="remoterealmcount",
+            name="zilencer_remoterealmcount_server_id_remote_id_de1573d8_idx",
+        ),
+    ]

zilencer/models.py

@@ -207,11 +207,14 @@ class RemoteInstallationCount(BaseRemoteCount):
                 condition=Q(subgroup__isnull=True),
                 name="unique_remote_installation_count_null_subgroup",
             ),
-        ]
-        indexes = [
-            models.Index(
+            UniqueConstraint(
                 fields=["server", "remote_id"],
-                name="zilencer_remoteinstallat_server_id_remote_id_f72e4c30_idx",
+                # As noted above, remote_id may be null, so we only
+                # enforce uniqueness if it isn't.  This is not
+                # technically necessary, since null != null, but it
+                # makes the property more explicit.
+                condition=Q(remote_id__isnull=False),
+                name="unique_remote_installation_count_server_id_remote_id",
             ),
         ]
 
@@ -243,16 +246,21 @@ class RemoteRealmCount(BaseRemoteCount):
                 condition=Q(subgroup__isnull=True),
                 name="unique_remote_realm_installation_count_null_subgroup",
             ),
+            UniqueConstraint(
+                fields=["server", "remote_id"],
+                # As with RemoteInstallationCount above, remote_id may
+                # be null; since null != null, this condition is not
+                # strictly necessary, but serves to make the property
+                # more explicit.
+                condition=Q(remote_id__isnull=False),
+                name="unique_remote_realm_installation_count_server_id_remote_id",
+            ),
         ]
         indexes = [
             models.Index(
                 fields=["property", "end_time"],
                 name="zilencer_remoterealmcount_property_end_time_506a0b38_idx",
             ),
-            models.Index(
-                fields=["server", "remote_id"],
-                name="zilencer_remoterealmcount_server_id_remote_id_de1573d8_idx",
-            ),
         ]
 
     @override