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/main.py

@@ -11,7 +11,7 @@ import os
 
 from app.config import settings
 from app.database import init_db
-from app.routers import photos, folders, heaps, tags, trash, library
+from app.routers import photos, folders, heaps, tags, discard, library
 from app.services.scanner import start_initial_scan
 
 # Configure logging
@@ -64,7 +64,7 @@ app.include_router(photos.router, prefix="/api/v1/photos", tags=["photos"])
 app.include_router(folders.router, prefix="/api/v1/folders", tags=["folders"])
 app.include_router(heaps.router, prefix="/api/v1/heaps", tags=["heaps"])
 app.include_router(tags.router, prefix="/api/v1/tags", tags=["tags"])
-app.include_router(trash.router, prefix="/api/v1/trash", tags=["trash"])
+app.include_router(discard.router, prefix="/api/v1/discard", tags=["discard"])
 app.include_router(library.router, prefix="/api/v1/library", tags=["library"])
 
 @app.get("/")