DEV: Partially move watched_precedence_over_muted migration to post-migration phase (#35755)

The change_column_null and change_column_default operations for the
watched_precedence_over_muted column have been moved from the
pre-migration
script to the post-migration script. This adjustment ensures that the
column
is only modified after all data transformations are complete, reducing
the
risk of data integrity issues during the migration process.

We have already seen this once in production, where a later failed
migration left the column in a bad state.

db/migrate/20251016013505_convert_watched_precedence_over_muted_to_default_pref.rb

@@ -25,9 +25,6 @@ class ConvertWatchedPrecedenceOverMutedToDefaultPref < ActiveRecord::Migration[8
       SET watched_precedence_over_muted = :preference_value
       WHERE watched_precedence_over_muted IS NULL;
     SQL
-
-    change_column_default :user_options, :watched_precedence_over_muted, from: nil, to: false
-    change_column_null :user_options, :watched_precedence_over_muted, false
   end
 
   def down

db/post_migrate/20251029072625_delete_old_watched_precedence_setting.rb

@@ -4,6 +4,9 @@ class DeleteOldWatchedPrecedenceSetting < ActiveRecord::Migration[8.0]
     DB.exec(<<~SQL)
       DELETE FROM site_settings WHERE name = 'watched_precedence_over_muted';
     SQL
+
+    change_column_default :user_options, :watched_precedence_over_muted, from: nil, to: false
+    change_column_null :user_options, :watched_precedence_over_muted, false
   end
 
   def down