Integrate group files: nullable room for uploads, archive class, claim idempotency, upload route

Wires the three parallel group-cloud tracks to one surface and fixes the
seams between them:

- chat_media_archive.room_id becomes nullable (direct web uploads have no
  Matrix room yet — the contract's null-event pattern extends to the room);
  core GroupFile.roomId and ArchiveEntryInput.roomId follow.
- mime_class gains 'archive' (zip) across the 029 CHECK, validation
  MIME_CLASSES and the bot classifier — one vocabulary everywhere.
- registerArchiveEntry claim keys are null-safe for room-less uploads.
- New: POST /files/groups/:groupId/upload — FileInterceptor (20 MiB) streams
  through magic-byte sniffing, library-safe naming, oCIS upload and the
  metadata index, behind group-membership ACL (Owner/Admin bypass).
- safeFileName/sniffMime promoted to packages/core (file-names.ts,
  file-mime.ts) so the bot and the api share one implementation; the bot's
  local modules become re-export shims; @types/node added to core.

Verified: api typecheck clean, group-files 22/22, bot 74/74, web typecheck
clean; 029 re-applied clean on a fresh scratch DB (room_id nullable,
archive class, claim_key unique all confirmed in the catalog).

packages/core/src/file-mime.ts

@@ -0,0 +1,44 @@
+export const MIME_CLASSES = ["image", "video", "audio", "document", "archive", "other"] as const;
+export type MimeClass = (typeof MIME_CLASSES)[number];
+
+export interface SniffedMime {
+  mime: string;
+  mimeClass: MimeClass;
+  extension: string | null;
+}
+
+/** Bytes needed to recognise every supported signature. */
+export const SNIFF_BYTES = 16;
+
+const UNKNOWN: SniffedMime = { mime: "application/octet-stream", mimeClass: "other", extension: null };
+
+function startsWith(head: Uint8Array, signature: readonly number[], offset = 0): boolean {
+  if (head.length < offset + signature.length) return false;
+  return signature.every((byte, i) => head[offset + i] === byte);
+}
+
+function ascii(text: string): number[] {
+  return [...text].map((c) => c.charCodeAt(0));
+}
+
+/**
+ * MIME is decided by magic bytes only; the sender's declared `info.mimetype`
+ * is never trusted. Unrecognised payloads are stored as octet-stream/other.
+ */
+export function sniffMime(head: Uint8Array): SniffedMime {
+  if (startsWith(head, [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])) {
+    return { mime: "image/png", mimeClass: "image", extension: "png" };
+  }
+  if (startsWith(head, [0xff, 0xd8, 0xff])) return { mime: "image/jpeg", mimeClass: "image", extension: "jpg" };
+  if (startsWith(head, ascii("GIF87a")) || startsWith(head, ascii("GIF89a"))) {
+    return { mime: "image/gif", mimeClass: "image", extension: "gif" };
+  }
+  if (startsWith(head, ascii("%PDF-"))) return { mime: "application/pdf", mimeClass: "document", extension: "pdf" };
+  if (startsWith(head, [0x50, 0x4b, 0x03, 0x04])) {
+    return { mime: "application/zip", mimeClass: "archive", extension: "zip" };
+  }
+  if (startsWith(head, ascii("ftyp"), 4)) return { mime: "video/mp4", mimeClass: "video", extension: "mp4" };
+  if (startsWith(head, [0x1a, 0x45, 0xdf, 0xa3])) return { mime: "video/webm", mimeClass: "video", extension: "webm" };
+  if (startsWith(head, ascii("OggS"))) return { mime: "audio/ogg", mimeClass: "audio", extension: "ogg" };
+  return UNKNOWN;
+}

packages/core/src/file-names.ts

@@ -0,0 +1,44 @@
+export const MAX_SAFE_NAME_BYTES = 120;
+const FALLBACK = "attachment";
+const WINDOWS_RESERVED = /^(con|prn|aux|nul|com[0-9]|lpt[0-9])(\..*)?$/i;
+
+function truncateUtf8(text: string, maxBytes: number): string {
+  let out = "";
+  for (const ch of text) {
+    if (Buffer.byteLength(out + ch, "utf8") > maxBytes) break;
+    out += ch;
+  }
+  return out;
+}
+
+/**
+ * Library-safe single path segment: no directories, control or reserved
+ * characters, leading dots, or unbounded length. Extension is preserved when
+ * truncating; a sniffed extension is appended when the name has none.
+ */
+export function safeFileName(raw: string, sniffedExtension: string | null = null): string {
+  const basename = raw.normalize("NFKC").split(/[/\\]/).pop() ?? "";
+  let name = basename
+    .replace(/[\p{Cc}\p{Cf}]/gu, "")
+    .replace(/[<>:"|?*#%{}^~[\]`]/g, "_")
+    .replace(/\s+/g, " ")
+    .replace(/_+/g, "_")
+    .trim()
+    .replace(/^[.\s_-]+/, "")
+    .replace(/[.\s]+$/, "");
+  if (name === "") name = FALLBACK;
+  if (WINDOWS_RESERVED.test(name)) name = `_${name}`;
+
+  const dot = name.lastIndexOf(".");
+  let stem = dot > 0 ? name.slice(0, dot) : name;
+  let ext = dot > 0 ? name.slice(dot + 1).toLowerCase() : "";
+  if (ext === "" && sniffedExtension !== null) ext = sniffedExtension;
+  if (Buffer.byteLength(ext, "utf8") > 16) {
+    stem = name;
+    ext = sniffedExtension ?? "";
+  }
+  const suffix = ext === "" ? "" : `.${ext}`;
+  stem = truncateUtf8(stem, MAX_SAFE_NAME_BYTES - Buffer.byteLength(suffix, "utf8")).trim();
+  if (stem === "") stem = FALLBACK;
+  return `${stem}${suffix}`;
+}

packages/core/src/index.ts

@@ -11,6 +11,8 @@ export * from "./capabilities.js";
 export * from "./chat-contract.js";
 export * from "./chat-contract.schema.js";
 export * from "./connection.js";
+export * from "./file-mime.js";
+export * from "./file-names.js";
 export * from "./messaging.js";
 export * from "./publishing.js";
 export * from "./shared-files.js";

packages/core/src/shared-files.ts

@@ -26,7 +26,8 @@ export interface GroupFile {
   id: string;
   workspaceId: string;
   groupId: string;
-  roomId: string;
+  /** Matrix room when the file arrived through one; null for direct uploads. */
+  roomId: string | null;
   senderId: string;
   /** Null for a direct upload/export until a link is posted in the room. */
   eventId: string | null;