Added transaction feature, fixed some mistakes

backend/migrations/000004_create_otp.up.sql

@@ -0,0 +1,17 @@
+CREATE UNLOGGED TABLE otp
+(
+    user_id    BIGINT    NOT NULL,
+    otp        TEXT      NOT NULL,
+    expired_at TIMESTAMP NOT NULL,
+
+    FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE CASCADE
+);
+
+CREATE INDEX idx_otp_user_id ON otp (user_id);
+CREATE INDEX idx_otp_expired_at ON otp (expired_at);
+
+SELECT cron.schedule(
+    'cleanup-expired-otp',
+    '*/10 * * * *',
+    $$DELETE FROM otp WHERE expired_at <= NOW()$$
+);