fix: respect an explicit false for notify_on_metadata_change

The after_initialize default unconditionally reset the flag to true on
new records, so an explicitly provided false (e.g. an unchecked form
checkbox) could never survive initialization. Only apply the default
when the attribute is nil, and add a regression test.

Addresses Copilot review on PR #72.

Author: dkastl

app/models/subscription_template.rb

@@ -71,7 +71,7 @@ def set_default_alteration_types
   end
 
   def set_default_notify_on_metadata_change
-    self.notify_on_metadata_change = true
+    self.notify_on_metadata_change = true if notify_on_metadata_change.nil?
   end
 
   def take_json_entities

test/unit/subscription_template_test.rb

@@ -36,6 +36,11 @@ def test_default_notify_on_metadata_change
     assert_equal true, template.notify_on_metadata_change
   end
 
+  def test_explicit_false_notify_on_metadata_change_is_respected
+    template = SubscriptionTemplate.new(valid_attributes(notify_on_metadata_change: false))
+    assert_equal false, template.notify_on_metadata_change
+  end
+
   def test_name_is_required
     template = SubscriptionTemplate.new(valid_attributes(name: nil))
     assert_not template.valid?