feat(server): support ai plan (#6216)

packages/backend/server/migrations/20240319104623_user_subscriptions/migration.sql

@@ -0,0 +1,11 @@
+/*
+  Warnings:
+
+  - A unique constraint covering the columns `[user_id,plan]` on the table `user_subscriptions` will be added. If there are existing duplicate values, this will fail.
+
+*/
+-- DropIndex
+DROP INDEX "user_subscriptions_user_id_key";
+
+-- CreateIndex
+CREATE UNIQUE INDEX "user_subscriptions_user_id_plan_key" ON "user_subscriptions"("user_id", "plan");