hvac: add device delete (retire stale mhi_modbus rows after MQTT switch)

There was no way to remove a zone_devices mapping — a gap exposed by the
Modbus->MQTT switchover, which leaves the old mhi_modbus rows stale beside
the freshly auto-registered mhi_mqtt ones. Adds DELETE /api/devices/:id
(manage_devices cap, unlinks photo files then deletes; device_photos
cascade) and a 'Delete device' button in the expanded device row.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/api.ts

@@ -71,6 +71,9 @@ export function discoverDevices(deviceType: string): Promise<{ ok: boolean; devi
 export function updateDevice(id: number, body: { zone_id?: number | null; location?: string }): Promise<Device> {
   return request(`/devices/${id}`, { method: 'PATCH', body: JSON.stringify(body) })
 }
+export function deleteDevice(id: number): Promise<{ ok: boolean }> {
+  return request(`/devices/${id}`, { method: 'DELETE' })
+}
 export function createMaintenanceAsset(id: number): Promise<{ ok: boolean; notImplemented?: boolean; error?: string }> {
   return request(`/devices/${id}/create-maintenance-asset`, { method: 'POST', body: JSON.stringify({}) })
 }