feat(routes): 迁移音频流代理与结果交付端点（流式）

- web/routers/stream.py：/api/v1/audio_stream 音频范围流代理
  · 签名校验（core.security，密钥同源两栈互认）；HEAD 直接 200 不请求上游
  · Range 原样透传上游（206 + Content-Range，拖动进度可用）
  · 编码透明：上游带 Content-Encoding 时不沿用压缩前长度（同 Flask F92）
  · StreamingResponse 16KB 分块，finally 关闭上游（下游中断亦释放）
- core/http_client.py 增 open()：返回未关闭流式响应，便于先读状态码再转发
- web/routers/delivery.py：/download/result、/download/batch_zip/result
  · 归属 403 / 进行中 202 / 暂停 200 / 取消 200 / 失败 500 / 缺失 404 分支逐一对齐
  · FileResponse 流式交付 + no-store + X-Download-Filename；清理由监控线程统一回收

验证：pytest 51/51（新增 5 项流代理：无效 token/HEAD/全量/Range 透传/上游关闭）；
ruff All checks passed；pyright 0 errors；奇偶校验 26/26；实测 5.2MB 流与
Range 206 正确。

web/app.py

@@ -22,9 +22,11 @@ from core.security import init_secret_key
 from web.errors import install_error_handlers
 from web.middleware import ResponseHygieneMiddleware
 from web.routers import account as account_router
+from web.routers import delivery as delivery_router
 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 stream as stream_router
 from web.routers import system as system_router
 from web.routers import tasks as tasks_router
 
@@ -75,6 +77,8 @@ def create_app(serve_flask_fallback: bool = False) -> FastAPI:
     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'])
+    app.include_router(delivery_router.router, prefix='/api/v1', tags=['delivery'])
+    app.include_router(stream_router.router, prefix='/api/v1', tags=['stream'])
 
     install_error_handlers(app)