fix(library/stats): exclude hidden photos from discarded count

The Discarded sidebar entry navigates to /photos?is_discarded=true, which
already excludes is_hidden=true rows (the cross-cutting hidden-folder
filter). The /library/stats discarded_count did not, so the badge could
disagree with the actual list — e.g. dtoro saw 1,281 in the badge but
only 25 in the view because the hidden Memories/ source root holds 1,256
discarded rows. Aligning the count with the view, like every other
sidebar badge already does.

backend/app/routers/library.py

@@ -85,7 +85,9 @@ async def get_library_stats(
 
     discarded_count = (
         await db.execute(
-            select(func.count(Photo.id)).where(owner, Photo.is_discarded.is_(True))
+            select(func.count(Photo.id)).where(
+                owner, Photo.is_hidden.is_(False), Photo.is_discarded.is_(True)
+            )
         )
     ).scalar() or 0