CLI red: backup tests, commander + env-paths deps, stub

4 tests for runBackup: full download into collection-named dirs,
incremental skip of existing files, resilient continuation after
single-file HTTP 500, and metadata.json output.

Adds commander 14.0.3 and env-paths 4.0.0 as runtime deps.

src/backup.ts

@@ -0,0 +1,25 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+import type { Client } from "./client.js";
+
+export interface BackupError {
+    fileID: number;
+    title: string;
+    collection: string;
+    error: string;
+}
+
+export interface BackupResult {
+    totalFiles: number;
+    downloaded: number;
+    skipped: number;
+    failed: number;
+    errors: BackupError[];
+}
+
+export const runBackup = async (
+    _client: Client,
+    _outDir: string,
+): Promise<BackupResult> => {
+    throw new Error("backup.runBackup not implemented");
+};