feat(shared): A6 任务协议契约、任务数据模型与系统配置

背景：A6 任务中心需要统一任务协议（九态状态机 / 队列映射 / SSE 事件）与任务落库模型，供 API、worker 与前端共用，避免各端自建枚举。
改动：
- 新增 packages/shared/src/tasks/task-protocol.ts：任务类型、架构 §3.6 九态与允许迁移、5 组 9 队列定义、Outbox/SSE 事件、进度与收敛纯函数；
- 新增 packages/shared/src/schemas/tasks.ts：提交/列表/详情/取消/重试/SSE 的 Zod DTO（body camelCase、query snake_case，列表 { items, next_cursor }）；
- system_configs 新增 task 分组（演练上限 20 项 / 单条成本 0.02 元 / 单条时长 3 秒），seed 按定义幂等；
- Prisma 新增 media_tasks / media_items / task_events 三表与迁移 20261104125003_a6_media_tasks（expand-only，不改旧表）。
影响面：仅新增 shared 导出、配置项与三张新表，旧接口与旧表不受影响。
回滚：撤销提交，并对三张新表执行 prisma migrate resolve/删除（无业务数据）。
验证：shared 197 tests 全绿；typecheck / lint / format:check 全绿；prisma migrate status up to date。

prisma/migrations/20261104125003_a6_media_tasks/migration.sql

@@ -0,0 +1,93 @@
+-- CreateTable
+CREATE TABLE "media_tasks" (
+    "id" CHAR(26) NOT NULL,
+    "type" VARCHAR(16) NOT NULL,
+    "status" VARCHAR(24) NOT NULL DEFAULT 'queued',
+    "title" VARCHAR(200) NOT NULL,
+    "customer_id" CHAR(26) NOT NULL,
+    "customer_name" VARCHAR(128) NOT NULL,
+    "created_by" CHAR(26) NOT NULL,
+    "created_by_name" VARCHAR(64) NOT NULL,
+    "item_total" INTEGER NOT NULL,
+    "item_succeeded" INTEGER NOT NULL DEFAULT 0,
+    "item_failed" INTEGER NOT NULL DEFAULT 0,
+    "item_skipped" INTEGER NOT NULL DEFAULT 0,
+    "estimated_items" INTEGER NOT NULL,
+    "estimated_cost" DECIMAL(14,4) NOT NULL,
+    "estimated_seconds" INTEGER NOT NULL,
+    "progress" INTEGER NOT NULL DEFAULT 0,
+    "payload_json" JSONB NOT NULL,
+    "cancel_requested_at" TIMESTAMPTZ(3),
+    "canceled_at" TIMESTAMPTZ(3),
+    "canceled_by" CHAR(26),
+    "last_error_code" VARCHAR(32),
+    "last_error_message" VARCHAR(500),
+    "started_at" TIMESTAMPTZ(3),
+    "finished_at" TIMESTAMPTZ(3),
+    "created_at" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ(3) NOT NULL,
+
+    CONSTRAINT "media_tasks_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "media_items" (
+    "id" CHAR(26) NOT NULL,
+    "task_id" CHAR(26) NOT NULL,
+    "seq" INTEGER NOT NULL,
+    "status" VARCHAR(24) NOT NULL DEFAULT 'queued',
+    "failure_code" VARCHAR(64),
+    "failure_message" VARCHAR(500),
+    "attempts" INTEGER NOT NULL DEFAULT 0,
+    "started_at" TIMESTAMPTZ(3),
+    "finished_at" TIMESTAMPTZ(3),
+    "created_at" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ(3) NOT NULL,
+
+    CONSTRAINT "media_items_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "task_events" (
+    "id" CHAR(26) NOT NULL,
+    "task_id" CHAR(26) NOT NULL,
+    "seq" BIGSERIAL NOT NULL,
+    "event_type" VARCHAR(16) NOT NULL,
+    "payload_json" JSONB NOT NULL,
+    "created_at" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "task_events_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "media_tasks_customer_id_created_at_idx" ON "media_tasks"("customer_id", "created_at" DESC);
+
+-- CreateIndex
+CREATE INDEX "media_tasks_status_created_at_idx" ON "media_tasks"("status", "created_at" DESC);
+
+-- CreateIndex
+CREATE INDEX "media_tasks_type_created_at_idx" ON "media_tasks"("type", "created_at" DESC);
+
+-- CreateIndex
+CREATE INDEX "media_tasks_created_by_created_at_idx" ON "media_tasks"("created_by", "created_at" DESC);
+
+-- CreateIndex
+CREATE INDEX "media_items_task_id_status_idx" ON "media_items"("task_id", "status");
+
+-- CreateIndex
+CREATE INDEX "media_items_status_idx" ON "media_items"("status");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "media_items_task_id_seq_key" ON "media_items"("task_id", "seq");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "task_events_seq_key" ON "task_events"("seq");
+
+-- CreateIndex
+CREATE INDEX "task_events_task_id_seq_idx" ON "task_events"("task_id", "seq");
+
+-- AddForeignKey
+ALTER TABLE "media_items" ADD CONSTRAINT "media_items_task_id_fkey" FOREIGN KEY ("task_id") REFERENCES "media_tasks"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "task_events" ADD CONSTRAINT "task_events_task_id_fkey" FOREIGN KEY ("task_id") REFERENCES "media_tasks"("id") ON DELETE CASCADE ON UPDATE CASCADE;