Rewrite the backup command on the library API with a durable failure ledger (closes #51)

Rewrites backup on the library API. backup() refreshes, fetches pending originals (and optionally thumbnails) through the pools reusing the content cache, then materialises the unchanged collections/ symlink views + per-collection JSON + sidecars from the model. A symlink failure no longer aborts the run (closes #8). failures.json reconciles against each run's attempted set — since-deleted/out-of-scope/resolved entries clear, still-failing retained, one attempt per file per run — and the exit-code contract is preserved.

Model: opus-4-8

bin/quak.ts

@@ -8,7 +8,7 @@ import { Command } from "commander";
 import envPaths from "env-paths";
 import { Client, type ClientSnapshot } from "../src/client.js";
 import { init } from "../src/crypto/index.js";
-import { runBackup } from "../src/backup.js";
+import { Library } from "../src/library/index.js";
 import { runMetadataBackup } from "../src/metadata-backup.js";
 import {
     listMissingThumbnails,
@@ -321,9 +321,14 @@ program
         const client = requireSession();
 
         stderr.write("Starting backup...\n");
-        const result = await runBackup(client, dir, (msg) => {
-            if (!opts.json) stderr.write(msg + "\n");
+        const lib = await Library.open({ client, downloadDirectory: dir });
+        const result = await lib.backup({
+            downloadDirectory: dir,
+            onProgress: (msg) => {
+                if (!opts.json) stderr.write(msg + "\n");
+            },
         });
+        lib.close();
 
         if (opts.json) {
             stdout.write(JSON.stringify(result, null, 2) + "\n");