refactor: rename trash to discard end-to-end

User-facing labels and code now use "discard" (verb) and "Discarded"
(state/view label) instead of "trash" / "Trashed". The DB column names
stay (is_trashed / trashed_at) so no migration is required — only the
SQLAlchemy attribute names are renamed via Column('old_name', ...).

Backend
- Photo model: is_discarded / discarded_at attributes (DB columns
  unchanged).
- PhotoBase / PhotoResponse / PhotoUpdate schemas use the new field
  names.
- Photos list endpoint: is_discarded query param, filter logic.
- DELETE /photos/{id} now sets is_discarded; success message updated.
- Bulk action 'trash' renamed to 'discard'.
- backend/app/routers/trash.py renamed to discard.py with renamed
  functions and route prefix /api/v1/discard.
- main.py imports and mounts the discard router.
- tasks/scan.py marks missing files as is_discarded.

Frontend
- Photo TS type: is_discarded.
- PhotoThumbnail: shows the trash-can icon when is_discarded.
- RightSidebar: button label "Discard"; mutation field name; local
  variable rename.
- TopBar: discardPhotosMutation and "Discard" button; toast text
  "Discarded".
- LeftSidebar: virtual node id 'discarded' / label "Discarded".
- FilterBar / filterStore / useFilterUrlSync: FlagFilter enum value
  'trashed' → 'discarded'; backend param key is_discarded.
- KeyboardHints: X label "Discard".
- useKeyboardShortcuts: PhotoUpdate field rename, X handler.
- api.ts: /trash routes → /discard, trash export → discard,
  bulkUpdate trash field → discard.

Out of scope (intentional): the docker-compose trash_data volume,
backend/Dockerfile mkdir /data/trash, config.py TrashSettings, and
the spec doc — all unused since soft-discard, and renaming them is
churn for no benefit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/TopBar.tsx

@@ -58,18 +58,18 @@ export function TopBar() {
   const selectedCount = selectedPhotos.length
   const queryClient = useQueryClient()
   
-  // Mutation for moving photos to trash
-  const trashPhotosMutation = useMutation({
+  // Mutation for discarding selected photos
+  const discardPhotosMutation = useMutation({
     mutationFn: async () => {
-      await photos.bulkUpdate(selectedPhotos, { trash: true })
+      await photos.bulkUpdate(selectedPhotos, { discard: true })
     },
     onSuccess: () => {
-      toast.success('Moved to Trash', `${selectedCount} photo${selectedCount > 1 ? 's' : ''} moved to trash`)
+      toast.success('Discarded', `${selectedCount} photo${selectedCount > 1 ? 's' : ''} discarded`)
       clearSelection()
       queryClient.invalidateQueries({ queryKey: ['photos'] })
     },
     onError: (error: any) => {
-      toast.error('Failed to Move to Trash', error.message || 'An error occurred')
+      toast.error('Failed to discard', error.message || 'An error occurred')
     },
   })
   
@@ -96,13 +96,13 @@ export function TopBar() {
               {selectedCount} selected
             </span>
             <button
-              onClick={() => trashPhotosMutation.mutate()}
-              disabled={trashPhotosMutation.isPending}
+              onClick={() => discardPhotosMutation.mutate()}
+              disabled={discardPhotosMutation.isPending}
               className="flex items-center gap-1 rounded bg-reject/20 px-2 py-0.5 text-sm text-reject hover:bg-reject/30 disabled:opacity-50"
-              title="Move to trash"
+              title="Discard"
             >
               <Trash2 className="h-3.5 w-3.5" />
-              Trash
+              Discard
             </button>
           </>
         )}