Convert refresh_tokens.token from VARCHAR to UUID

- Change token column type from VARCHAR(255) to UUID
- Add gen_random_uuid() as default value for token
- Improves type safety and performance for token storage

database/schema/schema.sql

@@ -36,7 +36,7 @@ CREATE TABLE users (
 CREATE TABLE refresh_tokens (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
     user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-    token VARCHAR(255) UNIQUE NOT NULL,
+    token UUID UNIQUE NOT NULL DEFAULT gen_random_uuid(),
     expires_at TIMESTAMP WITH TIME ZONE NOT NULL,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     revoked_at TIMESTAMP WITH TIME ZONE