0.32.1: backup/restore fixes (audit findings, all paths live-verified)

- backup_filename() crashed with NameError (bare now()) since 0.8.0 —
  broke every scheduled backup at the upload step, agent backup download
  and the central remote-backup/push endpoints. The local manual path
  worked only because the router had its own copy (now an alias).
- restore: the manifest stack_id from an uploaded backup is now slugified
  too — a crafted '../../...' id could previously escape STACKS_DIR.
- create_backup no longer starts a previously-stopped stack (stop/restart
  only when the stack was actually running).
- overwrite-restore wipes the existing volume contents before extracting,
  so files created since the backup no longer survive underneath it.

Verified end-to-end: full/config backup contents (compose, .env, .secrets,
bind dirs, extras, volume tars), delete→restore round-trip incl. volume
data, rename restore with volume re-prefixing, 409 conflict + overwrite,
traversal guard, scheduled run + retention prune + restore-from against
real MinIO, and the complete remote-agent cycle (download/push/restore).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.32.0",
+  "version": "0.32.1",
   "type": "module",
   "scripts": {
     "dev": "vite",