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/site/__init__.py

@@ -110,6 +110,8 @@ async def router_site_config(session: SessionDep) -> SiteConfigResponse:
 
     return SiteConfigResponse(
         title=s.get("siteName") or "DiskNext",
+        logo_light=s.get("logo_light") or None,
+        logo_dark=s.get("logo_dark") or None,
         register_enabled=s.get("register_enabled") == "1",
         login_captcha=s.get("login_captcha") == "1",
         reg_captcha=s.get("reg_captcha") == "1",