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.

frontend/src/types/index.ts

@@ -91,6 +91,11 @@ export interface Photo {
   url: string;
   thumbnail_url?: string;
   hero_url?: string; // Hero-optimized image URL (1920x1080) for full-width hero sections
+  // Lightbox preview URL (#492). Set only when the admin has flipped
+  // lightbox_preview_enabled in Settings → Thumbnails. Aspect-preserved
+  // ≤1920px JPEG; the lightbox prefers it over `url` for image photos
+  // and falls back to `url` when null (off, video, or not yet generated).
+  preview_url?: string | null;
   secure_url_template?: string;
   download_url_template?: string;
   requires_token?: boolean;