feat(storage): add file storage helpers module

Adds ensureStorageDir, saveFile, deleteStoredFile, and getStoredFilePath
utilities that manage the lifecycle of files on the local filesystem
under FILES_STORAGE_PATH.

apps/api/src/lib/file-storage.ts

@@ -0,0 +1,32 @@
+import { mkdir, writeFile, unlink } from "node:fs/promises";
+import { join, extname } from "node:path";
+import { randomUUID } from "node:crypto";
+import { env } from "../config.js";
+
+let storageReady = false;
+
+export async function ensureStorageDir(): Promise<void> {
+  if (storageReady) return;
+  await mkdir(env.FILES_STORAGE_PATH, { recursive: true });
+  storageReady = true;
+}
+
+export async function saveFile(buffer: Buffer, originalName: string): Promise<string> {
+  await ensureStorageDir();
+  const ext = extname(originalName).toLowerCase() || ".bin";
+  const storedName = `${randomUUID()}${ext}`;
+  await writeFile(join(env.FILES_STORAGE_PATH, storedName), buffer);
+  return storedName;
+}
+
+export async function deleteStoredFile(storedName: string): Promise<void> {
+  try {
+    await unlink(join(env.FILES_STORAGE_PATH, storedName));
+  } catch {
+    // File already gone
+  }
+}
+
+export function getStoredFilePath(storedName: string): string {
+  return join(env.FILES_STORAGE_PATH, storedName);
+}