feat(groups): assign tutor to group

db/migration.sql

@@ -154,6 +154,8 @@ CREATE TABLE IF NOT EXISTS user_branches (
   PRIMARY KEY (user_id, branch_id)
 );
 
+ALTER TABLE groups ADD COLUMN IF NOT EXISTS tutor_id INT REFERENCES users(id) ON DELETE SET NULL;
+
 CREATE TABLE IF NOT EXISTS sessions (
   id SERIAL PRIMARY KEY,
   user_id INT NOT NULL REFERENCES users(id) ON DELETE CASCADE,