fix(admin): test email always sends, regardless of update availability (#418)

The "Send Test Email" button on the Update Notifications settings page
called sendUpdateNotificationNow() — which bailed out with "No updates
available" when the instance was already on the latest version. Admins
on a current install had no way to verify their SMTP / recipient list
was working until an update happened to be pending. Reported in #418
by @Rekoo-PS.

Changes:

- Add migration 087: insert a dedicated `version_update_test` email
  template (EN + DE, matching the existing version_update_available
  convention) with copy that reads as a config-check rather than as a
  real update notice. Subject prefixed with [TEST] so it's unambiguous
  in the inbox. Variables: current_version, channel, recipient_email.

- Replace sendUpdateNotificationNow() with sendTestUpdateNotification()
  in updateNotificationService.js. The new path:
    - Always sends — no updateAvailable bail-out.
    - Uses the version_update_test template.
    - Falls back gracefully if checkForUpdates fails (so a transient
      GitHub API hiccup doesn't block a config-check email).
    - Does NOT update last_notified_version — that field stays owned by
      the real-update path so a test send doesn't shadow a future
      genuine notification for the same version.

- Wire /admin/system/updates/notifications/send to the renamed function.
  No frontend change needed (the button already calls this endpoint).

Verified locally with the dev mailhog: clicking Send Test Email on a
3.42.3-beta.0 instance (which has no pending update) delivers 4 emails
to all admin recipients with subject "[TEST] PicPeak Update Notification
— configuration check" and body interpolated correctly. Returns
{success: true, successCount: 4, ...} — previously would have returned
{success: false, message: "No updates available"}.

backend/migrations/core/087_add_update_notification_test_template.js

@@ -0,0 +1,130 @@
+/**
+ * Migration 087: Add a dedicated email template for the admin "Send Test
+ * Email" button on the Update Notifications settings page (#418).
+ *
+ * Previously the button reused the version_update_available template via
+ * sendUpdateNotificationNow(), which bailed out early when no real update
+ * was pending — so admins on the latest version had no way to verify
+ * their SMTP / recipient list config worked.
+ *
+ * The test template makes the intent unambiguous in the inbox ("This is
+ * a test of your update-notification setup, no action needed") and lets
+ * the send code run unconditionally regardless of update availability.
+ *
+ * Languages: EN + DE only, matching the convention of the existing
+ * version_update_available template (070). The email_templates table
+ * doesn't have nl/pt/ru columns; sendTemplateEmail falls back to EN.
+ */
+
+exports.up = async function(knex) {
+  console.log('Running migration: 087_add_update_notification_test_template');
+
+  const existing = await knex('email_templates')
+    .where('template_key', 'version_update_test')
+    .first();
+
+  if (existing) {
+    console.log('  version_update_test template already exists, skipping insert');
+    return;
+  }
+
+  await knex('email_templates').insert({
+    template_key: 'version_update_test',
+    subject_en: '[TEST] PicPeak Update Notification — configuration check',
+    subject_de: '[TEST] PicPeak Update-Benachrichtigung — Konfigurationsprüfung',
+    body_html_en: `
+<h2>This is a test email</h2>
+
+<p>You are receiving this message because an administrator clicked
+<strong>Send Test Email</strong> on the Update Notifications page of your
+PicPeak installation.</p>
+
+<div style="background-color: #f0f8ff; border-left: 4px solid #5C8762; padding: 20px; margin: 20px 0; border-radius: 4px;">
+  <p style="margin: 0;"><strong>Installed version:</strong> {{current_version}}</p>
+  <p style="margin: 10px 0 0 0;"><strong>Channel:</strong> {{channel}}</p>
+  <p style="margin: 10px 0 0 0;"><strong>Recipient address:</strong> {{recipient_email}}</p>
+</div>
+
+<p>If you can read this email, your SMTP configuration and the recipient
+list are working correctly. When a real new version becomes available,
+PicPeak will send a separate notification with release notes and update
+instructions.</p>
+
+<p style="color: #666; font-size: 13px; margin-top: 30px;">No action is required.
+You may safely delete this message.</p>
+
+<p>Best regards,<br>
+Your PicPeak Installation</p>`,
+    body_text_en: `This is a test email
+
+You are receiving this message because an administrator clicked
+"Send Test Email" on the Update Notifications page of your PicPeak
+installation.
+
+Installed version: {{current_version}}
+Channel: {{channel}}
+Recipient address: {{recipient_email}}
+
+If you can read this email, your SMTP configuration and the recipient
+list are working correctly. When a real new version becomes available,
+PicPeak will send a separate notification with release notes and update
+instructions.
+
+No action is required. You may safely delete this message.
+
+Best regards,
+Your PicPeak Installation`,
+    body_html_de: `
+<h2>Dies ist eine Test-E-Mail</h2>
+
+<p>Sie erhalten diese Nachricht, weil ein Administrator auf der Seite
+"Update-Benachrichtigungen" Ihrer PicPeak-Installation auf
+<strong>Test-E-Mail senden</strong> geklickt hat.</p>
+
+<div style="background-color: #f0f8ff; border-left: 4px solid #5C8762; padding: 20px; margin: 20px 0; border-radius: 4px;">
+  <p style="margin: 0;"><strong>Installierte Version:</strong> {{current_version}}</p>
+  <p style="margin: 10px 0 0 0;"><strong>Kanal:</strong> {{channel}}</p>
+  <p style="margin: 10px 0 0 0;"><strong>Empfänger-Adresse:</strong> {{recipient_email}}</p>
+</div>
+
+<p>Wenn Sie diese E-Mail lesen können, funktionieren Ihre SMTP-Konfiguration
+und die Empfängerliste korrekt. Sobald eine echte neue Version verfügbar
+ist, sendet PicPeak eine separate Benachrichtigung mit Versionshinweisen
+und Update-Anweisungen.</p>
+
+<p style="color: #666; font-size: 13px; margin-top: 30px;">Es ist keine Aktion
+erforderlich. Sie können diese Nachricht gefahrlos löschen.</p>
+
+<p>Mit freundlichen Grüßen,<br>
+Ihre PicPeak-Installation</p>`,
+    body_text_de: `Dies ist eine Test-E-Mail
+
+Sie erhalten diese Nachricht, weil ein Administrator auf der Seite
+"Update-Benachrichtigungen" Ihrer PicPeak-Installation auf
+"Test-E-Mail senden" geklickt hat.
+
+Installierte Version: {{current_version}}
+Kanal: {{channel}}
+Empfänger-Adresse: {{recipient_email}}
+
+Wenn Sie diese E-Mail lesen können, funktionieren Ihre SMTP-Konfiguration
+und die Empfängerliste korrekt. Sobald eine echte neue Version verfügbar
+ist, sendet PicPeak eine separate Benachrichtigung mit Versionshinweisen
+und Update-Anweisungen.
+
+Es ist keine Aktion erforderlich. Sie können diese Nachricht gefahrlos löschen.
+
+Mit freundlichen Grüßen,
+Ihre PicPeak-Installation`,
+    variables: JSON.stringify(['current_version', 'channel', 'recipient_email'])
+  });
+
+  console.log('Migration 087_add_update_notification_test_template completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 087_add_update_notification_test_template');
+  await knex('email_templates')
+    .where('template_key', 'version_update_test')
+    .del();
+};