feat(task): 任务中心扫码接单(多端) —— 服务端任务状态机/端口/迁移

- Task 加 task_code/tags/display_priority/claimed_by/claimed_at/doing_at；新增 TaskClaim 流水表。
- TaskService 加 claimed/doing/completed 状态机(claim/start_doing/complete)，grab 并入 claim。
- TaskRepository 加 list_published/get_by_code/update/claim/set_doing + TaskClaimRepository；挂 Database。
- 端口：
  · opc  /tasks/grab-by-code、/tasks/{id}/doing、/tasks/{id}/complete
  · operator POST /tasks(auto task_code) + PATCH /tasks/:id
  · park  /park/api/tasks(大屏展示,含 scan_payload 二维码载荷)
  · training /api/tasks/claim-by-code、/api/tasks/my(小程序 C 端账号→OPC 身份 find-or-create 领单)
- 迁移 0008(tasks 加列 + task_claims)已应用+stamp；seed 补 task_code/tags/grab demo。
- tests/test_task_claim.py(自包含内存库, 状态机+流水+list_published), 直接 async 校验通过。

Co-Authored-By: Claude <noreply@anthropic.com>

app/park/routers.py

@@ -11,12 +11,16 @@ import json
 import logging
 import os
 import time
+from datetime import datetime
 from pathlib import Path
 
-from fastapi import APIRouter, File, Header, HTTPException, Request, UploadFile
+from fastapi import APIRouter, Depends, File, Header, HTTPException, Request, UploadFile
 from fastapi.responses import FileResponse, JSONResponse, StreamingResponse
 from pydantic import BaseModel, Field
+from sqlalchemy.ext.asyncio import AsyncSession
 
+from ..infrastructure.db import get_session
+from ..infrastructure.repositories import TaskRepository
 from . import park_config, tenants
 from .auth import create_device_token, create_token, parse_device_token, parse_token, require_tenant
 from .config import settings
@@ -447,6 +451,41 @@ async def dashboard_overview(authorization: str | None = Header(None), tenant_id
     return get_engine(tid).snapshot()
 
 
+@router.get("/api/tasks", summary="大屏任务展示（平台全局已发布任务）")
+async def park_tasks(
+    authorization: str | None = Header(None),
+    tenant_id: str | None = None,
+    session: AsyncSession = Depends(get_session),
+):
+    """返回系统任务中心的上墙任务（published/claimed/doing），供大屏轮播卡片。
+
+    大屏卡片含任务 ID、task_code 与二维码（scan_payload）。任务为平台全局实体，
+    不受园区隔离；此处按园区身份校验后返回。
+    """
+    await _resolve_tenant(authorization, tenant_id)
+    repo = TaskRepository(session)
+    items = await repo.list_published()
+    now_label = datetime.now().strftime("%Y%m%d")
+
+    def payload(t: dict) -> dict:
+        return {
+            "id": t["id"],
+            "task_code": t["task_code"] or f"TK-{now_label}-{t['id'][-5:]}",
+            "scan_payload": t["task_code"] or f"TK-{now_label}-{t['id'][-5:]}",
+            "title": t["title"],
+            "category": t["category"],
+            "summary": (t["description"] or "")[:160],
+            "budget_min": t["budget_min"],
+            "budget_max": t["budget_max"],
+            "status": t["status"],
+            "claimed_by": t["claimed_by"],
+            "claimed_at": t["claimed_at"],
+            "tags": [tag for tag in (t.get("tags") or "").split(",") if tag],
+        }
+
+    return {"items": [payload(t) for t in items]}
+
+
 @router.get("/api/park/zones")
 async def park_zones(authorization: str | None = Header(None), tenant_id: str | None = None):
     tid = await _resolve_tenant(authorization, tenant_id)
@@ -500,8 +539,10 @@ def _gen_code() -> str:
 
 
 def _publish_bind(device_id: str, payload: dict) -> None:
-    """未绑定大屏仅订阅 bind 频道；绑定成功后经此频道通知并下发 token。"""
-    hub.publish(f"opc/display/bind/{device_id}", payload, qos=1)
+    """未绑定大屏仅订阅 bind 频道；绑定成功后经此频道通知并下发 token。
+    retain=True：绑定/解绑状态持久，屏幕即使延迟连接，订阅时也会立即收到最新状态（避免
+    绑定成功但屏幕 MQTT 尚未订阅导致丢失 bound 事件、无法自动进入）。"""
+    hub.publish(f"opc/display/bind/{device_id}", payload, qos=1, retain=True)
 
 
 @router.post("/api/devices/register")
@@ -545,6 +586,9 @@ async def bind_screen_by_code(tid: str, body: BindByCodeBody):
 async def device_unbind(device_id: str, authorization: str | None = Header(None), tenant_id: str | None = None):
     await _resolve_tenant(authorization, tenant_id)
     ok = await tenants.unbind_device(device_id)
+    if ok:
+        # 通知大屏退出到待绑定页（大屏订阅 bind/<device> 频道，收到 unbound 即清理本地并回绑定页）
+        _publish_bind(device_id, {"event": "unbound"})
     return {"ok": ok, "device_id": device_id}