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/hooks/useKeyboardShortcuts.ts

@@ -14,7 +14,7 @@ interface KeyboardShortcutsProps {
 interface PhotoUpdate {
   rating?: number
   is_picked?: boolean
-  is_trashed?: boolean
+  is_discarded?: boolean
   color_label?: string | null
 }
 
@@ -104,24 +104,24 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
 
   useHotkeys('0', () => updateActive({ rating: 0 }), HK_OPTS)
 
-  // Pick / trash / unflag. Trash is the merged "rejected" concept — a soft
-  // flag that hides the photo from the default timeline view; restore via
-  // the trash view (or the U shortcut).
+  // Pick / discard / unflag. Discard is a soft flag that hides the photo
+  // from the default timeline view; restore via the Discarded view (or the
+  // U shortcut).
   useHotkeys(
     'p',
-    () => updateActive({ is_picked: true, is_trashed: false }),
+    () => updateActive({ is_picked: true, is_discarded: false }),
     HK_OPTS
   )
 
   useHotkeys(
     'x',
-    () => updateActive({ is_trashed: true, is_picked: false }),
+    () => updateActive({ is_discarded: true, is_picked: false }),
     HK_OPTS
   )
 
   useHotkeys(
     'u',
-    () => updateActive({ is_picked: false, is_trashed: false }),
+    () => updateActive({ is_picked: false, is_discarded: false }),
     HK_OPTS
   )