zilencer: Update remoterealm indexes.

There is no reason to have an index on just `realm_id` or `remote_id`,
as those values mean nothing outside of the scope of a specific
`server_id`.  Remove those never-used single-column indexes from the
two tables that have them.

By contrast, the pair of `server_id` and `remote_id` is quite useful
and specific -- it is a unique pair, and every POST of statistics from
a remote host requires looking up the highest `remote_id` for a given
`server_id`, which (without this index) is otherwise a quite large
scan.

Add a unique constraint, which (in PostgreSQL) is implemented as a
unique index.

zilencer/migrations/0029_update_remoterealm_indexes.py

@@ -0,0 +1,45 @@
+# Generated by Django 4.2.5 on 2023-09-13 05:02
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zilencer", "0028_rename_extradatajson_remoteauditlog_extra_data"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="remoterealmauditlog",
+            name="realm_id",
+            field=models.IntegerField(),
+        ),
+        migrations.AlterField(
+            model_name="remoterealmauditlog",
+            name="remote_id",
+            field=models.IntegerField(),
+        ),
+        migrations.AlterField(
+            model_name="remoterealmcount",
+            name="realm_id",
+            field=models.IntegerField(),
+        ),
+        migrations.AlterField(
+            model_name="remoterealmcount",
+            name="remote_id",
+            field=models.IntegerField(),
+        ),
+        migrations.AddConstraint(
+            model_name="remoterealmauditlog",
+            constraint=models.UniqueConstraint(
+                fields=("server", "remote_id"), name="zilencer_remoterealmauditlog_server_remote"
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="remoterealmauditlog",
+            index=models.Index(
+                fields=["server", "realm_id", "remote_id"],
+                name="zilencer_remoterealmauditlog_server_realm_remote",
+            ),
+        ),
+    ]