fix(park): 大屏前端契约对齐 — update_check download_url=/download、media 返回 relative_path

大屏 api.js 以 getApiBase()(=/park) + 相对路径拼接，download_url 应 '/' 相对、media 应 relative_path。

app/park/routers.py

@@ -309,7 +309,7 @@ async def update_check(body: UpdateCheckBody):
     latest = m.group(1) if m else ""
     current = (body.current or "").strip()
     update_available = bool(latest) and (not current or current != latest)
-    return {"ok": True, "latest_version": latest, "installer_name": install, "download_url": "/park/download", "update_available": update_available}
+    return {"ok": True, "latest_version": latest, "installer_name": install, "download_url": "/download", "update_available": update_available}
 
 
 # ==================== 媒体 / 播放列表（storage → serverdata/park/data.json） ====================
@@ -319,7 +319,7 @@ def _media_files() -> list[dict]:
     out = []
     for f in sorted(settings.MEDIA_DIR.glob("*")):
         if f.is_file() and f.suffix.lower() in {".mp4", ".mkv", ".avi", ".jpg", ".jpeg", ".png"}:
-            out.append({"path": f"/park/file/{f.name}", "name": f.name,
+            out.append({"relative_path": f.name, "name": f.name,
                         "type": "video" if f.suffix.lower() in {".mp4", ".mkv", ".avi"} else "image",
                         "size": f.stat().st_size})
     return out