Add orders.created_at column

db/schema.sql

@@ -48,6 +48,7 @@ CREATE TABLE markets(
 
 CREATE TABLE orders(
     id SERIAL PRIMARY KEY,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
     market_id INTEGER NOT NULL REFERENCES markets(id),
     user_id INTEGER NOT NULL REFERENCES users(id),
     quantity BIGINT NOT NULL,