Fix: apply migration that enables the General topic feature

Patches the database migration for the General topic (#130) so it applies cleanly. PostgreSQL's CREATE OR REPLACE cannot change a function's return type, so the migration was blocked at apply time — adding is_general to the list_topics_for_space return set hit error 42P13 and rolled the whole migration back. The migration now drops and recreates that function, clearing the blocker and allowing the General topic to go live.