quak backup refuses to run while another backup of the same directory runs, exit 2 (closes #169)

lib.backup() takes a lock, backup.lock in its download directory, made
with proper-lockfile, before its refresh, and removes it when it ends. A
second backup of the directory fails at once with an error naming it.
quak backup takes the lock itself before it opens its library and passes
lockHeld to the backup, so a refused run sends no request; it prints the
error as one line and exits 2. A lock untouched for 10 seconds, left by a
run that could not remove it, is taken over.

Deviation: yarn.lock was regenerated by yarn add in the pinned node image.
Judgement call: a run failing at its refresh leaves the directory, empty.
Judgement call: a lock removed mid-run stops that run with an uncaught
error, the library's default.

Model: opus-5-5

src/library/index.ts

@@ -94,6 +94,7 @@ import type { Collection, EnteFile } from "../model/types.js";
 import { runBackup, type BackupOptions, type BackupResult } from "../backup.js";
 
 export {
+    lockBackupDirectory,
     runBackup,
     type BackupOptions,
     type BackupResult,
@@ -566,13 +567,16 @@ export class Library {
 
     // Back up every in-scope file to `opts.downloadDirectory`, or else the
     // library's, each original at its save path, with a durable failure
-    // ledger (issue #51). Waits for a completed refresh first, as `fresh()`
-    // does, joining one already running, and rejects before touching any file
-    // when it fails. Then puts pending originals at their save paths as
-    // `Photo.download()` does (and optional thumbnails) through the content
-    // cache and pools, waits for an ML data fetch, and rebuilds the derived
-    // symlink/JSON views from the model, each file's JSON with its ML data,
-    // beside an `account.json` with the account's email and user ID.
+    // ledger (issue #51). Takes the lock in that directory first, unless
+    // `opts.lockHeld` says the caller holds it, failing at once while another
+    // backup of it runs (see `runBackup`). Waits for a
+    // completed refresh, as `fresh()` does, joining one already running, and
+    // rejects before touching any file but the lock when it fails. Then puts
+    // pending originals at their save paths as `Photo.download()` does (and
+    // optional thumbnails) through the content cache and pools, waits for an
+    // ML data fetch, and rebuilds the derived symlink/JSON views from the
+    // model, each file's JSON with its ML data, beside an `account.json` with
+    // the account's email and user ID.
     // Throws before any network work when no content cache backs the
     // originals it must fetch.
     backup(opts?: BackupOptions): Promise<BackupResult> {