fix(backups,files): auto-backups rejected by validator; trip file download broken after cookie migration

Fixes #773: isValidBackupFilename regex anchored to ^backup- rejected all
auto-backup-* filenames, causing 400 on download/restore/delete. Broadened
to ^(?:auto-)?backup-.

Fixes #774: three regressions in the trip Files tab —
- openFile import shadowed by a local function of the same name inside
  FileManager; PDF preview modal was calling the local with a URL string,
  corrupting state and crashing on the second click (mime_type read on
  undefined). Fixed by aliasing the import as openFileUrl.
- GET /:id/download used a bespoke authenticateDownload that checked only
  Bearer header and ?token= query param, ignoring the trek_session cookie.
  After the JWT-to-cookie migration the client sends cookies only, so every
  download silently 401-ed. Extended authenticateDownload to accept req and
  check cookie → Bearer → query token in priority order.
- files.download and files.openError translation keys were missing from all
  15 locale files; t() was returning the raw key as a truthy string,
  defeating the || 'Download' fallback.

client/src/i18n/translations/de.ts

@@ -1224,6 +1224,8 @@ const de: Record<string, string | { name: string; category: string }[]> = {
   'files.title': 'Dateien',
   'files.pageTitle': 'Dateien & Dokumente',
   'files.subtitle': '{count} Dateien für {trip}',
+  'files.download': 'Herunterladen',
+  'files.openError': 'Datei konnte nicht geöffnet werden',
   'files.downloadPdf': 'PDF herunterladen',
   'files.count': '{count} Dateien',
   'files.countSingular': '1 Datei',