feat(devices): live device-status footer across all categories

Generalise printer-only monitoring to every configured device. New
DeviceMonitor polls all enabled devices each tick (default 8s): printers
via rich readStatus(), relays/readers/cameras via the generic healthCheck()
reachability probe, flattened to one traffic-light (ready/degraded/offline)
+ detail, deduped (emit on change only), fail-toward-offline.

- device-status bus event + GET /api/devices/status snapshot.
- Pushed over the existing /api/ws (hello carries the initial set;
  device-status frame per change).
- Web: live-store devices map, WS handler, DeviceFooter chip-per-device
  (role label not vendor; click a degraded/offline chip for an issues panel).

Verified roleKind resolution + change-only emit on a fresh DB.

Note: the footer's UI surface (api type, router mount, i18n devices) rides
in the subsequent subscription commit due to shared-file overlap.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

wiki/concepts/printer-status-monitoring.md

@@ -7,6 +7,11 @@ updated: 2026-06-14
 
 # Printer status monitoring
 
+> **Generalised 2026-06-18:** the booth's all-device status **footer** is a separate, unified
+> monitor across every category (relays/readers/cameras/printers) — see
+> [[device-status-monitoring]]. This page remains the authority for the *printer-specific* rich
+> status (paper/cover/cutter) + its SSE stream; both monitors run.
+
 The booth must know a printer is in trouble **before** a driver presses the entry button and no
 ticket comes out. So the system polls each printer's live status (paper out, cover open, cutter
 jam, off-line) and pushes changes to the operator UI. A reliability control, like