backup-metadata: keep going when an ML data request fails (closes #101)

Each ML data request of up to 200 files is now tried on its own. A request
that still fails after its retries is logged, its files are written with the
reason in `mlDataError`, and the command exits 1 once the dump is complete.
`fetchMLData`, used only here, is removed in favour of a per-batch loop over
`fetchMLDataBatch`.

Model: opus-5-5

src/cli-commands.ts

@@ -323,11 +323,11 @@ export const backupMetadataCommand = async (
     if (!client) return 1;
     const lib = await openReadLibrary(ctx, client);
     try {
-        await runMetadataBackup(lib, client, dir, {
+        const { failedMLBatches } = await runMetadataBackup(lib, client, dir, {
             exif: opts.exif || opts.all,
             onProgress: (msg) => ctx.stderr.write(msg + "\n"),
         });
-        return 0;
+        return failedMLBatches > 0 ? 1 : 0;
     } finally {
         await lib.close();
     }