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/src/routes/adminSystem.js

@@ -11,7 +11,7 @@ const { checkForUpdates, getCurrentChannel } = require('../services/updateCheckS
 const { detectEnvironment, generateUpdateInstructions } = require('../services/environmentService');
 const {
   checkAndNotifyUpdates,
-  sendUpdateNotificationNow,
+  sendTestUpdateNotification,
   getUpdateNotificationSettings
 } = require('../services/updateNotificationService');
 const router = express.Router();
@@ -352,13 +352,18 @@ router.put('/updates/notifications', adminAuth, requirePermission('settings.edit
 });
 
 // Manually trigger update notification email
+// Send a test update notification email. Uses the dedicated
+// `version_update_test` template (migration 087) rather than reusing
+// `version_update_available`, so admins on the latest version can still
+// verify their SMTP + recipient config — the previous handler bailed
+// with "No updates available" when nothing was pending (#418).
 router.post('/updates/notifications/send', adminAuth, requirePermission('settings.edit'), async (req, res) => {
   try {
-    const result = await sendUpdateNotificationNow();
+    const result = await sendTestUpdateNotification();
     res.json(result);
   } catch (error) {
-    logger.error('Error sending update notification:', error);
-    res.status(500).json({ error: 'Failed to send update notification' });
+    logger.error('Error sending test update notification:', error);
+    res.status(500).json({ error: 'Failed to send test update notification' });
   }
 });