Add index on BaseTarget.created to speed up load time of large DBs

We currently sort on the created field of BaseTarget, which causes
performance issues when there are a large number of BaseTargets.

In a database with 6 million rows, this reduced the time for the SQL
count(*) from ~800ms to ~100ms, significantly speeding up page load
time.

Note that this does NOT solve #598 but it does help a little.

tom_targets/base_models.py

@@ -299,7 +299,8 @@ class BaseTarget(models.Model):
     )
     created = models.DateTimeField(
         auto_now_add=True, verbose_name='Time Created',
-        help_text='The time which this target was created in the TOM database.'
+        help_text='The time which this target was created in the TOM database.',
+        db_index=True
     )
     modified = models.DateTimeField(
         auto_now=True, verbose_name='Last Modified',

tom_targets/migrations/0023_alter_basetarget_created.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.18 on 2025-02-05 21:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tom_targets', '0022_persistentshare'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='basetarget',
+            name='created',
+            field=models.DateTimeField(auto_now_add=True, db_index=True, help_text='The time which this target was created in the TOM database.', verbose_name='Time Created'),
+        ),
+    ]