feat: implement source link endpoints and enforce policy rules

- Add POST/GET source link endpoints for file sharing via permanent URLs
- Enforce max_size check in PATCH /file/content to prevent size limit bypass
- Support is_private (proxy) vs public (302 redirect) storage modes
- Replace all ResponseBase(data=...) with proper DTOs or 204 responses
- Add 18 integration tests for source link and policy rule enforcement

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routers/api/v1/slave/__init__.py

@@ -20,15 +20,15 @@ slave_aria2_router = APIRouter(
     summary='测试用路由',
     description='Test route for checking connectivity.',
 )
-def router_slave_ping() -> ResponseBase:
+def router_slave_ping() -> str:
     """
     Test route for checking connectivity.
-    
+
     Returns:
-        ResponseBase: A response model indicating success.
+        str: 后端版本号
     """
     from utils.conf.appmeta import BackendVersion
-    return ResponseBase(data=BackendVersion)
+    return BackendVersion
 
 @slave_router.post(
     path='/post',