feat(automatic-exchange-rates): add "Single exchange rate" where only one exchange rate is added and updated to avoid db clutter

app/apps/currencies/migrations/0015_exchangerate_automatic_exchangerateservice_singleton.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.2.4 on 2025-08-08 02:18
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('currencies', '0014_alter_currency_options'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='exchangerate',
+            name='automatic',
+            field=models.BooleanField(default=False, verbose_name='Automatic'),
+        ),
+        migrations.AddField(
+            model_name='exchangerateservice',
+            name='singleton',
+            field=models.BooleanField(default=False, help_text='Create one exchange rate and keep updating it. Avoids database clutter.', verbose_name='Single exchange rate'),
+        ),
+    ]