feat(routes): 迁移任务状态端点（topen/queue/batch_progress/task/*）

- web/routers/tasks.py：topen、queue、batch_progress、task/cancel、
  task/heartbeat、task/list 迁至 /api/v1/*
- 读写 core.state 共享任务表/统计（与 Flask 同一批对象）——两栈迁移期看到同一份
  任务状态，这是单进程双框架成立的关键
- task/cancel 保持 POST-only（与 Flask 同口径，杜绝顶层导航 GET 触发取消）

契约奇偶校验：24 项全 OK（新增 queue/topen/task:list/heartbeat/progress404）；
验证：pytest 46/46；ruff All checks passed；pyright 0 errors；冒烟 10/10。

web/app.py

@@ -26,6 +26,7 @@ from web.routers import music as music_router
 from web.routers import podcast as podcast_router
 from web.routers import site as site_router
 from web.routers import system as system_router
+from web.routers import tasks as tasks_router
 
 
 @asynccontextmanager
@@ -73,6 +74,7 @@ def create_app(serve_flask_fallback: bool = False) -> FastAPI:
     app.include_router(music_router.router, prefix='/api/v1', tags=['music'])
     app.include_router(account_router.router, prefix='/api/v1', tags=['account'])
     app.include_router(podcast_router.router, prefix='/api/v1', tags=['podcast'])
+    app.include_router(tasks_router.router, prefix='/api/v1', tags=['tasks'])
 
     install_error_handlers(app)