fix(host): derive template backup dir from config, drop hardcoded path (W2)

backup_template used a hardcoded _BACKUP_BASE_DIR=/var/lib/ci/backups; now derives <config.paths.root>/backups (matches the CLI --backup-base-dir default) per the plan's no-hardcoded-path gotcha. Tests no longer need to monkeypatch the constant.

plans/web-ui-management-plan.md

@@ -20,7 +20,7 @@ Stato passi (spunta man mano):
 
 - [x] **W0** — Scaffolding: extra `[web]` in `pyproject.toml`, dir package `operations/` + `web/`, skeleton test. Nessuna logica.
 - [x] **W1** — Layer `operations/vms.py`: estrai la logica da `commands/vm.py` in funzioni tipate che ritornano dataclass. Refactor dei comandi CLI per chiamarle. **Comportamento CLI invariato.** (Incluso: seam `VmBackend.list_running()` + fold del fix §2.8 IP-pool leak via `IpSlotPool.reconcile` / `operations.vms.reconcile_ip_leases`.)
-- [ ] **W2** — Layer `operations/cache.py` + `operations/host.py`: stato cache cargo, purge, disco, runner, backup template. Refactor `retention.py` / `monitor.py`.
+- [x] **W2** — Layer `operations/cache.py` + `operations/host.py`: stato cache cargo, purge, disco, runner, backup template. Refactor `retention.py` (`monitor.py` lasciato invariato — refactor opzionale; `disk_status`/`runner_status` verranno consumati dalla web app in W4).
 - [ ] **W3** — `operations/audit.py`: append-only JSONL + lettura. Config `[web]` in `config.py`.
 - [ ] **W4** — App web core: `web/app.py` factory + `commands/web.py` (`ci web serve`). Dashboard **read-only** (VM, cache, disco, runner). Ancora **senza auth**, bind localhost.
 - [ ] **W5** — Auth: Gitea OIDC (Authlib) + sessione firmata + allowlist utenti. Tutte le route protette.