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>

backend/app/models/photos.py

@@ -33,9 +33,10 @@ class Photo(Base):
     added_at = Column(DateTime, server_default=func.now())
     updated_at = Column(DateTime, onupdate=func.now())
     
-    # Trash status
-    is_trashed = Column(Boolean, default=False)
-    trashed_at = Column(DateTime)
+    # Discard status. The DB column names stay is_trashed/trashed_at to avoid
+    # a migration; only the Python attribute name reflects the rename.
+    is_discarded = Column('is_trashed', Boolean, default=False)
+    discarded_at = Column('trashed_at', DateTime)
     
     # Thumbnail paths
     thumb_small = Column(String)  # path to 240px thumb
@@ -55,7 +56,7 @@ class Photo(Base):
     rating = Column(Integer, default=0)  # 0-5 stars
     color_label = Column(String)  # 'red'|'orange'|'yellow'|'green'|'blue'|'purple'|NULL
     is_picked = Column(Boolean, default=False)
-    # Note: is_rejected was merged into is_trashed (a single soft "trashed"
+    # Note: is_rejected was merged into is_discarded (a single soft "discarded"
     # concept). The DB column may still exist on legacy installs but is no
     # longer read or written.