Fix (#13214): split mailrule maximum_age clamp into its own migration (#13231)

Avoids a Postgres error where the clamp UPDATE and the following ALTER TABLE on paperless_mail_mailrule share a transaction, but the table's FKs are deferrable, so pending trigger events block the ALTER. 

Also fixes a verbose_name mismatch in migration 0013.

src/paperless/migrations/0013_applicationconfiguration_llm_request_timeout.py

@@ -17,7 +17,7 @@ class Migration(migrations.Migration):
             field=models.PositiveSmallIntegerField(
                 null=True,
                 validators=[django.core.validators.MinValueValidator(1)],
-                verbose_name="Sets the LLM request timeout in seconds",
+                verbose_name="Sets the LLM timeout in seconds",
             ),
         ),
     ]