some changes?

db/002_subscribers.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS `subscribers` (
+  `subscriber_id` INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+  `user_id` INTEGER NOT NULL,
+  `kind` VARCHAR(16) NOT NULL,
+  `arguments` JSON NOT NULL DEFAULT '{}',
+  FOREIGN KEY(`user_id`)
+    REFERENCES users(user_id)
+      ON DELETE CASCADE
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS subscribers_kind_user_id
+  ON subscribers (`kind`, `user_id`);