feat(attach): 附件可配置限额、流式落盘与分阶段上传

管理端可配类型/体积；上传改流式避免整文件进内存；发帖先选再传并拦截未上传提交；提高 Next proxy 体积上限。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/components/MarkdownEditor.tsx

@@ -47,6 +47,8 @@ type MarkdownEditorProps = {
   disabled?: boolean;
   onSubmitShortcut?: () => void;
   id?: string;
+  /** 正文插图体积上限（MB），缺省 5 */
+  imageMaxMB?: number;
 };
 
 type EditorMode = "split" | "write" | "preview";
@@ -107,6 +109,7 @@ export default function MarkdownEditor({
   disabled,
   onSubmitShortcut,
   id,
+  imageMaxMB = 5,
 }: MarkdownEditorProps) {
   const taRef = useRef<HTMLTextAreaElement>(null);
   const fileRef = useRef<HTMLInputElement>(null);
@@ -227,8 +230,9 @@ export default function MarkdownEditor({
       toast("仅支持 JPEG / PNG / WebP", "error");
       return;
     }
-    if (file.size > 5 * 1024 * 1024) {
-      toast("图片不能超过 5MB", "error");
+    const maxBytes = Math.max(1, imageMaxMB) * 1024 * 1024;
+    if (file.size > maxBytes) {
+      toast(`图片不能超过 ${imageMaxMB}MB`, "error");
       return;
     }
     setUploading(true);