refactor(db): Use JSON format and add missing FK to colorscheme data

Author: reobin

db/migrations/20251005195216_colorschemes.sql

@@ -7,9 +7,9 @@ CREATE TABLE colorschemes (
 
 CREATE TABLE colorscheme_color_data (
   colorscheme_name TEXT NOT NULL,
-  repository_id INTEGER NOT NULL,
+  repository_id INTEGER NOT NULL REFERENCES repositories(id) ON DELETE CASCADE,
   background TEXT NOT NULL CHECK (background IN ('light', 'dark')),
-  color_data TEXT NOT NULL,
+  color_data JSON NOT NULL,
 
   PRIMARY KEY (colorscheme_name, repository_id, background)
 )