fix(preview): generate lightbox previews for external/reference photos (#1078) (#1080)

* fix(preview): generate lightbox previews for external/reference photos (#1078)

Stable twin of the main-line fix. ensurePreviewImage() resolved its source
only via resolvePhotoStorageKey(), which returns null for external/reference
photos by design — those live on a media mount outside the managed storage
tree. The null went straight into withLocalCopy(), which throws, so the
preview route fell back to redirecting at the full-size original. Galleries
whose photos are all external got no benefit from the preview tier (#492):
guests paid 5-12 MB on every lightbox open.

Add the external branch ensureThumbnail() already has: resolve via
resolvePhotoFilePath() and feed the mount path to generatePreviewImage()
directly, with an ext<id>_ output basename.

generatePreviewImage() on this branch hardcoded path.basename(imagePath) and
ignored options.outputBasename, so it needs the same one-line honouring that
generateThumbnail() already does — without it two events referencing the same
NAS basename collide on one preview key.

Also return null rather than throwing for a row with no source_origin in a
reference-mode event, whose mode falls back to the event's.

Claude-Session: https://claude.ai/code/session_01Ra4hcsYiKuQLbbRsg6EjAc

* fix(preview): select the columns the external branch needs on bulk regenerate

POST /api/admin/thumbnails/regenerate-previews selected only id, event_id,
path, media_type, mime_type and preview_path, so photo.source_origin was
undefined by the time ensurePreviewImage branched on it. Every external row in
a reference gallery took the managed path, resolvePhotoStorageKey returned null
for it, and the endpoint reported success while generating nothing.

Add source_origin, external_relpath and filename to the select, plus a
source-inspection test pinning the caller contract and a service-level test
showing a column-starved row is indistinguishable from a managed one.

Claude-Session: https://claude.ai/code/session_01Ra4hcsYiKuQLbbRsg6EjAc

---------

Co-authored-by: Paul Nothaft <[email protected]>

backend/src/routes/adminThumbnails.js

@@ -201,7 +201,13 @@ router.post('/regenerate-previews', adminAuth, requirePermission('photos.edit'),
   try {
     const { eventId } = req.body;
 
-    let query = db('photos').select('id', 'event_id', 'path', 'media_type', 'mime_type', 'preview_path');
+    // source_origin/external_relpath/filename are what ensurePreviewImage
+    // branches on for external/reference rows (#1078) — without them every
+    // external photo looks managed here and generation is skipped.
+    let query = db('photos').select(
+      'id', 'event_id', 'path', 'media_type', 'mime_type', 'preview_path',
+      'source_origin', 'external_relpath', 'filename'
+    );
     if (eventId) query = query.where('event_id', eventId);
     // Skip videos — preview tier is image-only.
     query = query.where(function() {