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/lib/fileKind.ts

@@ -0,0 +1,49 @@
+/** 附件类型徽章分类（详情页与发帖编辑共用） */
+
+export type FileKind = "image" | "archive" | "doc" | "media" | "file";
+
+export function fileKind(name: string, mime: string): FileKind {
+  const m = (mime || "").toLowerCase();
+  const ext = (name.split(".").pop() || "").toLowerCase();
+  if (
+    m.startsWith("image/") ||
+    ["png", "jpg", "jpeg", "gif", "webp", "svg", "bmp", "avif", "ico", "heic"].includes(ext)
+  ) {
+    return "image";
+  }
+  if (
+    m.includes("zip") ||
+    m.includes("rar") ||
+    m.includes("7z") ||
+    m.includes("compress") ||
+    ["zip", "rar", "7z", "tar", "gz", "bz2", "xz", "tgz"].includes(ext)
+  ) {
+    return "archive";
+  }
+  if (
+    m.startsWith("audio/") ||
+    m.startsWith("video/") ||
+    ["mp3", "wav", "flac", "aac", "ogg", "mp4", "mkv", "webm", "mov", "avi"].includes(ext)
+  ) {
+    return "media";
+  }
+  if (
+    m.includes("pdf") ||
+    m.includes("word") ||
+    m.includes("text") ||
+    m.includes("sheet") ||
+    m.includes("excel") ||
+    ["pdf", "doc", "docx", "txt", "md", "xls", "xlsx", "ppt", "pptx", "csv", "json"].includes(ext)
+  ) {
+    return "doc";
+  }
+  return "file";
+}
+
+export function fileExtBadge(name: string): string {
+  const i = name.lastIndexOf(".");
+  if (i <= 0 || i === name.length - 1) return "FILE";
+  const ext = name.slice(i + 1).replace(/[^a-zA-Z0-9]/g, "");
+  if (!ext || ext.length > 5) return "FILE";
+  return ext.toUpperCase();
+}