firewall: interface-coverage apply guard, target drift, non-destructive DHCP sync

Post-DHCP-incident hardening per HARDEN.md.

- apply guard: refuse (ConflictError, `force` overrides) when a
  network-managed interface would end up in no zone; absent
  `interfaces` key = hands-off, explicit `[]` = unassign-all
- surface `uncovered_interfaces` in firewall state (lo/wg* filtered)
  + advisory in /api/status/pending; zones.js banner + interfaces-picker
  last-zone confirm
- target drift (Option A): absent or default-normalizing target is
  unmanaged: not diffed, never re-set by apply; create_zone runs
  --new-zone first and sets non-default targets only; importer omits
  the target key for default zones
- FirewallToDhcpSync keeps stale DHCP ranges and flags them instead of
  deleting; `dnsmasq` affected only on a real gateway mutation
- real pre-apply recovery snapshot in data/firewall/rules.json
  ({timestamp, default_zone, zones, config}); drop the empty post-apply
  skeleton
- daemon shutdown: bounded grace for in-flight tasks + suppressed
  teardown exception noise on SIGTERM
- also carries the firewall service-descriptions feature
  (get_service_descriptions + service_descriptions state field + UI)
- tests + docs across firewall/status/state/sync/schema; ruff clean,
  867 passing

docs/overview.md

@@ -74,7 +74,7 @@ After installation, access the management interface at `https://<hostname>.local
 │   ├── nginx/sites-enabled/ # Generated server blocks
 │   ├── dnsmasq/fragments/   # User config fragments
 │   ├── acme/                # ACME certificates
-│   ├── firewall/            # Firewall rule backup
+│   ├── firewall/            # Pre-apply recovery snapshot
 │   ├── logs/                # Application logs
 │   ├── networkd/            # Generated 50-<name>.network files
 │   └── wireguard/           # Generated WireGuard configs