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/routers/discard.py

@@ -0,0 +1,50 @@
+"""
+Discard API router
+"""
+from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import select, and_
+from sqlalchemy.ext.asyncio import AsyncSession
+from datetime import datetime
+
+from app.database import get_db
+from app.models import Photo
+
+router = APIRouter()
+
+@router.get("")
+async def list_discarded(db: AsyncSession = Depends(get_db)):
+    """List discarded photos"""
+    result = await db.execute(
+        select(Photo).where(Photo.is_discarded == True)
+    )
+    photos = result.scalars().all()
+    return photos
+
+@router.post("/restore")
+async def restore_photos(photo_ids: list[str], db: AsyncSession = Depends(get_db)):
+    """Restore photos from the discard pile"""
+    result = await db.execute(
+        select(Photo).where(and_(Photo.id.in_(photo_ids), Photo.is_discarded == True))
+    )
+    photos = result.scalars().all()
+
+    for photo in photos:
+        photo.is_discarded = False
+        photo.discarded_at = None
+
+    await db.commit()
+    return {"status": "success", "restored": len(photos)}
+
+@router.delete("/empty")
+async def empty_discard(db: AsyncSession = Depends(get_db)):
+    """Permanently delete all discarded photos"""
+    result = await db.execute(
+        select(Photo).where(Photo.is_discarded == True)
+    )
+    photos = result.scalars().all()
+
+    for photo in photos:
+        await db.delete(photo)
+
+    await db.commit()
+    return {"status": "success", "deleted": len(photos)}