fix(admin/exports): Lightroom TXT export joins with comma + drops extension (#623)

The PhotoExportMenu's TXT format advertises "Simple text list for Lightroom
search" but emitted newline-separated filenames WITH `.jpg`. Lightroom's
filename search wants a comma-separated one-liner, and the gallery JPEGs may
correspond to RAW files in the catalog — so the search has to match on the
stem only.

The frontend now passes `separator: 'comma'` + `include_extension: false` for
the TXT format specifically. The backend gains an `include_extension` option
(defaulting to true so direct API consumers don't break), and the comma case
joins without a trailing space (the form Lightroom expects). Unit test pins
the Lightroom-mode output AND the backward-compatible default for any direct
API caller.

CSV / XMP / JSON exports are unchanged.

frontend/src/components/admin/PhotoExportMenu.tsx

@@ -64,6 +64,11 @@ export const PhotoExportMenu: React.FC<PhotoExportMenuProps> = ({
       format,
       options: {
         filename_format: 'original',
+        // TXT is labelled "for Lightroom search" — Lightroom's filename
+        // search field takes one comma-separated line, and the gallery
+        // JPEGs may correspond to RAW files in the catalog so the search
+        // has to match on the stem only (issue #623).
+        ...(format === 'txt' ? { separator: 'comma' as const, include_extension: false } : {}),
         include_rating: true,
         include_label: true,
         include_description: true,