feat: add visibility column migration to replace is_public

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Author: fullstackjam

migrations/0006_add_visibility.sql

@@ -0,0 +1,7 @@
+-- Add visibility column (public, unlisted, private) to replace is_public
+ALTER TABLE configs ADD COLUMN visibility TEXT DEFAULT 'unlisted';
+
+-- Migrate existing data: is_public=1 -> 'public', is_public=0 -> 'private'
+UPDATE configs SET visibility = 'public' WHERE is_public = 1;
+UPDATE configs SET visibility = 'private' WHERE is_public = 0;
+UPDATE configs SET visibility = 'unlisted' WHERE is_public IS NULL;