fix(slideshow): stop "no crop" fit letterboxing a pre-cropped frame (stable) (#1015) (#1019)

Backport of #1018 to the curated channel; the reporter on #1015 is running v3.45.14.

The slideshow resolved its image as preview_url || hero_url || url. preview_url is only emitted when lightbox_preview_enabled is on (default false), so a default install fell through to hero_url — the 1920x1080 fit:'cover' centre crop built for gallery header banners. object-fit: contain then letterboxed an already-cropped 16:9 frame.

Emits slideshow_url (same aspect-preserved preview tier) unconditionally for image photos; the show prefers it and never falls back to hero_url. preview_url stays gated so the lightbox opt-in is unchanged.

backend/src/routes/gallery.js

@@ -726,6 +726,17 @@ router.get('/:slug/photos', verifyGalleryAccess, resolveGuest, async (req, res)
             && (!photo.mime_type || !photo.mime_type.startsWith('video/'))
             ? `/api/gallery/${req.params.slug}/preview/${photo.id}${wmQuery}`
             : null,
+          // Slideshow source (#1015). Same preview tier, but emitted
+          // unconditionally: the slideshow has no `url` fallback worth
+          // taking (originals are projector-sized) and must never land on
+          // `hero_url`, which is cover-cropped to 16:9 — that made the
+          // "no crop" fit letterbox an already-cropped frame. The preview
+          // route generates lazily and redirects to the original on any
+          // failure, so this is safe even where no preview exists yet.
+          slideshow_url: photo.media_type !== 'video'
+            && (!photo.mime_type || !photo.mime_type.startsWith('video/'))
+            ? `/api/gallery/${req.params.slug}/preview/${photo.id}${wmQuery}`
+            : null,
           secure_url_template: `/api/secure-images/${req.params.slug}/secure/${photo.id}/{{token}}`,
           download_url_template: `/api/secure-images/${req.params.slug}/secure-download/${photo.id}/{{token}}`,
           type: photo.type,