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>

src/Service/DeviceSerializer.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use App\Entity\Device;
+
+/**
+ * Single source of truth for the wire shape of a Device. The REST API uses it
+ * to render `/api/devices` responses; the Mercure publisher uses it so live
+ * pushes are byte-identical to a fresh GET, letting the SPA splat the payload
+ * straight into its store.
+ */
+final class DeviceSerializer
+{
+    /** @return array<string, mixed> */
+    public function serialize(Device $d): array
+    {
+        return [
+            'id'                      => $d->getId(),
+            'mac'                     => $d->getMac(),
+            'name'                    => $d->getName(),
+            'orientation'             => $d->getOrientation()->value,
+            'rotationIntervalMinutes' => $d->getRotationIntervalMinutes(),
+            'wakeTimes'               => $d->getWakeTimes(),
+            'timezone'                => $d->getTimezone(),
+            'uniquenessWindow'        => $d->getUniquenessWindow(),
+            'linkedAt'                => $d->getLinkedAt()->format(\DateTimeInterface::ATOM),
+            'lastSeenAt'              => $d->getLastSeenAt()?->format(\DateTimeInterface::ATOM),
+            'nextPollExpectedAt'      => $d->getNextPollExpectedAt()?->format(\DateTimeInterface::ATOM),
+            'lockedImageId'           => $d->getLockedImage()?->getId(),
+            'currentImageId'          => $d->getCurrentImage()?->getId(),
+        ];
+    }
+}

src/Service/MercurePublisher.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Service;
+
+use Psr\Log\LoggerInterface;
+use Symfony\Component\Mercure\HubInterface;
+use Symfony\Component\Mercure\Update;
+
+/**
+ * Thin wrapper around the Mercure hub for device-state pushes.
+ *
+ * Topic shape: `https://pictureframe.edholm.me/devices/{id}` — same convention
+ * aqua-iq uses. The browser subscribes per device id; the published payload is
+ * the same JSON the REST API would have returned, so the SPA can splat it
+ * straight into the device store with no separate envelope handling.
+ *
+ * Errors are swallowed and logged — a flaky hub must never break a poll
+ * response or a settings PATCH for the user.
+ */
+final class MercurePublisher
+{
+    public const TOPIC_PREFIX = 'https://pictureframe.edholm.me/devices/';
+
+    public function __construct(
+        private readonly HubInterface    $hub,
+        private readonly LoggerInterface $logger,
+    ) {}
+
+    public function publishDevice(int $deviceId, array $payload): void
+    {
+        try {
+            $this->hub->publish(new Update(
+                self::TOPIC_PREFIX . $deviceId,
+                json_encode($payload, JSON_THROW_ON_ERROR),
+            ));
+        } catch (\Throwable $e) {
+            $this->logger->warning('mercure.publish_failed', [
+                'device_id' => $deviceId,
+                'error'     => $e->getMessage(),
+            ]);
+        }
+    }
+}