feat(server): 站内通知基础设施与关键事件触发(F14,F30)

- 新增 notifications 表与 API(list/mark-read)
- submit/approve/reject/return/transfer/pay/forgot-password 写入站内通知
- 通知失败降级，不阻塞主流程

server/prisma/migrations/20260209091354_add_notifications_table/migration.sql

@@ -0,0 +1,23 @@
+-- CreateTable
+CREATE TABLE "notifications" (
+    "id" UUID NOT NULL,
+    "recipient_employee_id" UUID NOT NULL,
+    "type" VARCHAR(64) NOT NULL,
+    "title" VARCHAR(200) NOT NULL,
+    "body" TEXT,
+    "target_type" VARCHAR(64),
+    "target_id" UUID,
+    "read_at" TIMESTAMP(3),
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "notifications_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "notifications_recipient_employee_id_created_at_idx" ON "notifications"("recipient_employee_id", "created_at");
+
+-- CreateIndex
+CREATE INDEX "idx_notifications_unread_query" ON "notifications"("recipient_employee_id", "read_at", "created_at");
+
+-- AddForeignKey
+ALTER TABLE "notifications" ADD CONSTRAINT "notifications_recipient_employee_id_fkey" FOREIGN KEY ("recipient_employee_id") REFERENCES "employees"("id") ON DELETE CASCADE ON UPDATE CASCADE;