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/RightSidebar.tsx

@@ -27,7 +27,7 @@ interface PhotoDetails {
   taken_at: string | null
   rating: number
   is_picked: boolean
-  is_trashed: boolean
+  is_discarded: boolean
   exif_json: string | null
 }
 
@@ -108,7 +108,7 @@ export function RightSidebar() {
     mutationFn: (data: {
       rating?: number
       is_picked?: boolean
-      is_trashed?: boolean
+      is_discarded?: boolean
     }) => photosApi.update(activePhotoId!, data),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['photo', activePhotoId] })
@@ -132,7 +132,7 @@ export function RightSidebar() {
   const multipleSelected = selectedPhotos.length > 1
   const rating = photo?.rating ?? 0
   const isPicked = photo?.is_picked ?? false
-  const isTrashed = photo?.is_trashed ?? false
+  const isDiscarded = photo?.is_discarded ?? false
 
   return (
     <div className="flex h-full flex-col bg-surface">
@@ -187,7 +187,7 @@ export function RightSidebar() {
                 onClick={() =>
                   updateMutation.mutate({
                     is_picked: !isPicked,
-                    is_trashed: false,
+                    is_discarded: false,
                   })
                 }
                 className={clsx(
@@ -203,19 +203,19 @@ export function RightSidebar() {
               <button
                 onClick={() =>
                   updateMutation.mutate({
-                    is_trashed: !isTrashed,
+                    is_discarded: !isDiscarded,
                     is_picked: false,
                   })
                 }
                 className={clsx(
                   'flex items-center gap-1 rounded px-2 py-1 text-sm transition-colors',
-                  isTrashed
+                  isDiscarded
                     ? 'bg-reject/20 text-reject'
                     : 'bg-surface-2 text-text-muted hover:bg-surface-offset'
                 )}
               >
                 <Trash2 className="h-3 w-3" />
-                Trash
+                Discard
               </button>
             </div>
           </div>