fix(attachments): Use dynamic retention for AddField, static 30d for AlterField

In migration 1060, use default_attachment_retention() for the initial
AddField so existing rows get an expiry based on the environment's
configured retention. Immediately follow with an AlterField to set the
permanent db_default to a static NOW() + 30 days.

Update migration 1061 to carry the same static 30-day expression
instead of the old epoch sentinel.

Author: jan-auer

src/sentry/migrations/1060_eventattachment_date_expires.py

@@ -7,6 +7,11 @@
 from django.db import migrations, models
 
 from sentry.new_migrations.migrations import CheckedMigration
+from sentry.objectstore import default_attachment_retention
+
+# Initial default applies to existing attachments. New rows will be written with
+# a runtime-defined default, so we follow-up with a static 30d default.
+_RETENTION_DAYS = default_attachment_retention()
 
 
 class Migration(CheckedMigration):
@@ -30,6 +35,17 @@ class Migration(CheckedMigration):
 
     operations = [
         migrations.AddField(
+            model_name="eventattachment",
+            name="date_expires",
+            field=models.DateTimeField(
+                db_default=django.db.models.expressions.CombinedExpression(
+                    django.db.models.functions.datetime.Now(),
+                    "+",
+                    models.Value(datetime.timedelta(days=_RETENTION_DAYS)),
+                ),
+            ),
+        ),
+        migrations.AlterField(
             model_name="eventattachment",
             name="date_expires",
             field=models.DateTimeField(

src/sentry/migrations/1061_eventattachment_date_expires_index.py

@@ -1,6 +1,9 @@
 # Generated by Django 5.2.12 on 2026-04-01 12:13
 
 import datetime
+
+import django.db.models.expressions
+import django.db.models.functions.datetime
 from django.db import migrations, models
 
 from sentry.new_migrations.migrations import CheckedMigration
@@ -30,7 +33,11 @@ class Migration(CheckedMigration):
             model_name="eventattachment",
             name="date_expires",
             field=models.DateTimeField(
-                db_default=datetime.datetime(1970, 1, 1, 0, 0, tzinfo=datetime.timezone.utc),
+                db_default=django.db.models.expressions.CombinedExpression(
+                    django.db.models.functions.datetime.Now(),
+                    "+",
+                    models.Value(datetime.timedelta(days=30)),
+                ),
                 db_index=True,
             ),
         ),