Add index on succeeded tasks (#164)

* Add index on task table

* also add models file

* add make commands and limit index

* format

django_q/migrations/0018_task_success_index.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.2.7 on 2024-03-05 17:03
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("django_q", "0017_task_cluster_alter"),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name="task",
+            index=models.Index(
+                condition=models.Q(("success", True)),
+                fields=["group", "name", "func"],
+                name="success_index",
+            ),
+        ),
+    ]