feat(lightbox): medium-resolution preview tier (#492)

Adds an opt-in lightbox preview tier so guests open photos against an
aspect-preserved ~1920px JPEG (~200–500 KB) instead of the full original
(often 5–12 MB). Originals are still served on Download.

Backend:
  - imageProcessor: generatePreviewImage / isPreviewValid / ensurePreviewImage
    using fit:'inside' + withoutEnlargement (longEdge 1920, q85, mozjpeg)
  - migration 104: photos.preview_path + lightbox_preview_enabled setting
    (off by default, JSON-stringified for SQLite/Postgres parity)
  - GET /api/gallery/:slug/preview/:photoId — gallery-auth, lazy generation,
    ETag based on mtime+photoId+watermarkHash
  - preview_url surfaced in the photo response only when the toggle is on
  - admin /thumbnails/regenerate-previews mirrors regenerate-thumbnails,
    skipping videos
  - backup walk + archive cleanup + photo-delete now include previews/

Frontend:
  - PhotoLightbox uses photo.preview_url ?? photo.url (null-safe fallback)
  - ThumbnailsTab gets a Lightbox Preview Tier card: opt-in toggle +
    Regenerate All Previews button (gated until the toggle is on)
  - en/de locale strings; nl/pt/ru/fr fall back to en

Tested end-to-end: 11 MB / 4000×3000 source → 985 KB / 1920×1440 preview,
381 ms first call, 7 ms cached, ~91% byte reduction.

backend/src/services/archiveService.js

@@ -129,7 +129,10 @@ async function archiveEvent(event) {
       );
     }
 
-    // Delete thumbnails for this event's photos.
+    // Delete derived images (thumbnails / heroes / previews / watermarks)
+    // for this event's photos. The originals are inside the zip; the
+    // derived tiers are throwaway and will be regenerated lazily on
+    // restore (or not at all for archived events that nobody opens).
     const photos = await db('photos').where('event_id', event.id);
     for (const photo of photos) {
       if (photo.thumbnail_path) {
@@ -138,6 +141,11 @@ async function archiveEvent(event) {
       if (photo.hero_path) {
         await storage.delete(photo.hero_path).catch(() => {});
       }
+      // Lightbox preview tier (#492). Same disposable-derived
+      // semantics as thumbnails / heroes — wipe on archive.
+      if (photo.preview_path) {
+        await storage.delete(photo.preview_path).catch(() => {});
+      }
       // Best effort: remove watermarked variants too if a refactor added them.
       if (photo.watermark_path) {
         await storage.delete(photo.watermark_path).catch(() => {});