settings: Add require_e2ee_push_notifications realm setting.

This commit adds a realm setting:
"Require end-to-end encryption for push notification content".

This commit just involves backend & UI changes to add the setting,
the impact of this setting is in the next commit.

Fixes part of #35370.

zerver/migrations/0743_realm_require_e2ee_push_notifications.py

@@ -0,0 +1,38 @@
+# Generated by Django 5.2.4 on 2025-07-28 18:58
+
+from django.conf import settings
+from django.db import migrations, models
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def update_require_e2ee_push_notifications(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    Realm = apps.get_model("zerver", "Realm")
+
+    # We use 'getattr' with a default value to allow this migration
+    # to run in development environment when PUSH_NOTIFICATION_REDACT_CONTENT
+    # setting is removed in the future.
+    require_e2ee = getattr(settings, "PUSH_NOTIFICATION_REDACT_CONTENT", False)
+    if require_e2ee:
+        Realm.objects.update(require_e2ee_push_notifications=require_e2ee)
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0742_usermessage_zerver_usermessage_is_private_unread_message_id"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="realm",
+            name="require_e2ee_push_notifications",
+            field=models.BooleanField(db_default=False, default=False),
+        ),
+        migrations.RunPython(
+            update_require_e2ee_push_notifications,
+            elidable=True,
+            reverse_code=migrations.RunPython.noop,
+        ),
+    ]