perf(jobs): run preview pre-warm daily, time-insensitive

lib/BackgroundJob/PreviewEnhancementProcessingJob.php

@@ -35,8 +35,9 @@ public function __construct(
 		$this->userManager = $userManager;
 		$this->jobList = $jobList;
 
-		$this->setInterval(3600);
-		$this->setTimeSensitivity(self::TIME_SENSITIVE);
+		// Background pre-warm only; previews are computed on demand on mailbox open, so run rarely and defer under load
+		$this->setInterval(24 * 3600);
+		$this->setTimeSensitivity(self::TIME_INSENSITIVE);
 	}
 
 	/**

tests/Unit/BackgroundJob/PreviewEnhancementProcessingJobTest.php

@@ -0,0 +1,56 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Mail\Tests\Unit\BackgroundJob;
+
+use ChristophWurst\Nextcloud\Testing\TestCase;
+use OCA\Mail\BackgroundJob\PreviewEnhancementProcessingJob;
+use OCA\Mail\Service\AccountService;
+use OCA\Mail\Service\PreprocessingService;
+use OCP\AppFramework\Utility\ITimeFactory;
+use OCP\BackgroundJob\IJobList;
+use OCP\IUserManager;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+
+class PreviewEnhancementProcessingJobTest extends TestCase {
+
+	private ITimeFactory&MockObject $time;
+	private IUserManager&MockObject $userManager;
+	private AccountService&MockObject $accountService;
+	private PreprocessingService&MockObject $preprocessingService;
+	private LoggerInterface&MockObject $logger;
+	private IJobList&MockObject $jobList;
+	private PreviewEnhancementProcessingJob $job;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->time = $this->createMock(ITimeFactory::class);
+		$this->userManager = $this->createMock(IUserManager::class);
+		$this->accountService = $this->createMock(AccountService::class);
+		$this->preprocessingService = $this->createMock(PreprocessingService::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+		$this->jobList = $this->createMock(IJobList::class);
+
+		$this->job = new PreviewEnhancementProcessingJob(
+			$this->time,
+			$this->userManager,
+			$this->accountService,
+			$this->preprocessingService,
+			$this->logger,
+			$this->jobList,
+		);
+	}
+
+	public function testRunsDailyAndDeferrableUnderLoad(): void {
+		self::assertSame(24 * 3600, $this->job->getInterval());
+		self::assertFalse($this->job->isTimeSensitive());
+	}
+}