feat(home): live updates via Mercure — server pushes device state to the PWA

Subscribe per-device with a Symfony Mercure hub: server publishes a fresh
device payload after every poll (200/304/204), every PATCH, and every
lock/unlock. The frontend opens one EventSource per device topic and
splats inbound JSON straight into the devices store — same shape as
GET /api/devices, so no envelope handling.

Topic: https://pictureframe.edholm.me/devices/{id}

Stack mirrors aqua-iq:
- symfony/mercure-bundle + config/packages/mercure.yaml
- App\Service\MercurePublisher (errors swallowed + logged; a flaky hub
  must not break a poll response)
- App\Service\DeviceSerializer extracted as the single source of truth
  for the wire shape (REST + Mercure share it)
- Frontend useDeviceMercure() composable: opens/closes EventSources to
  match the device list reactively, reconnects on hub-side closes
- SpaController exposes MERCURE_PUBLIC_URL via window.__PF_MERCURE_URL__

Production compose adds a dunglas/mercure container with Traefik labels
for pictureframe.edholm.me/.well-known/mercure (handled separately on
the host since the file isn't in this repo).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -208,6 +208,7 @@ import { computed, onMounted, onUnmounted, ref } from 'vue'
 import { useRouter } from 'vue-router'
 import { useDevicesStore } from '@/stores/devices'
 import { useUploadStore } from '@/stores/upload'
+import { useDeviceMercure } from '@/composables/useDeviceMercure'
 import type { Device } from '@/types'
 
 // Sync interval for status comparisons. Devices configured with explicit wake
@@ -330,6 +331,10 @@ const router       = useRouter()
 const devicesStore = useDevicesStore()
 const uploadStore  = useUploadStore()
 
+// Live updates: server publishes a fresh device payload after every poll
+// (and on PATCH/lock/unlock); the composable splats it into the store.
+useDeviceMercure()
+
 onMounted(() => {
   devicesStore.fetchDevices()
   document.addEventListener('visibilitychange', onVisibility)