test: close coverage gaps from the recent rotation + Mercure work

Frontend (90.15→95.37 stmts / 91.83→97.01 lines):
  - useDeviceMercure: full composable test suite via a fake EventSource —
    open/merge/ignore-stale/parse-error/reconnect/dynamic-add/remove/
    no-op-when-unconfigured/cleanup-on-unmount.
  - HomeView: cover onTimePart's AM/PM and minute branches plus the
    nextPollExpectedAt-null fallback paths in the next-update preview.

Backend (no instrumentation before; pcov was already in the image,
just needed a <coverage> block in phpunit.dist.xml):
  - RotationService: one test per mode (NewestUpload, Random,
    LeastRecentlyShown), one for never-shown sorting first under LRS,
    and two for prioritizeNeverShown — narrows when never-shown exists,
    falls through to mode otherwise.
  - DeviceSerializer: contract test on the wire shape (REST + Mercure
    use the same serializer; silent rename here would break live updates
    instantly).
  - MercurePublisher: topic format + JSON encoding + the swallow-
    exceptions guarantee (a flaky hub must not break poll responses).

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

tests/Unit/Service/MercurePublisherTest.php

@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Service;
+
+use App\Service\MercurePublisher;
+use PHPUnit\Framework\TestCase;
+use Psr\Log\NullLogger;
+use Symfony\Component\Mercure\HubInterface;
+use Symfony\Component\Mercure\Update;
+
+/**
+ * MercurePublisher's contract is small but load-bearing:
+ *   1. The topic format includes the device id (the SPA subscribes by id).
+ *   2. The payload is published as JSON.
+ *   3. A throwing hub MUST NOT propagate — a flaky Mercure container can
+ *      never break a poll response or a settings PATCH for the user.
+ */
+class MercurePublisherTest extends TestCase
+{
+    public function test_publishes_to_device_topic_with_json_payload(): void
+    {
+        $captured = null;
+        $hub = $this->createMock(HubInterface::class);
+        $hub->expects($this->once())
+            ->method('publish')
+            ->willReturnCallback(function (Update $u) use (&$captured) {
+                $captured = $u;
+                return 'urn:uuid:test';
+            });
+
+        $publisher = new MercurePublisher($hub, new NullLogger());
+        $publisher->publishDevice(42, ['id' => 42, 'name' => 'Living Room']);
+
+        $this->assertNotNull($captured);
+        $this->assertSame(
+            ['https://pictureframe.edholm.me/devices/42'],
+            $captured->getTopics(),
+        );
+        $this->assertSame('{"id":42,"name":"Living Room"}', $captured->getData());
+    }
+
+    public function test_swallows_hub_exceptions_so_callers_never_blow_up(): void
+    {
+        $hub = $this->createMock(HubInterface::class);
+        $hub->method('publish')->willThrowException(new \RuntimeException('hub down'));
+
+        $publisher = new MercurePublisher($hub, new NullLogger());
+
+        // No exception should escape — if this throws, the test fails.
+        $publisher->publishDevice(42, ['id' => 42]);
+        $this->expectNotToPerformAssertions();
+    }
+}