feat: per-photo permanent delete + discarded thumbnail treatment

Discarded photos now look discarded in the grid (50% opacity + grayscale)
with a red trash badge in the corner instead of a bare icon. The discard
action bar gains a "Delete N" button that permanently deletes only the
current selection, complementing the existing "Empty discard pile".

Backend: new DELETE /discard endpoint accepting {photo_ids: [...]} that
permanently removes only listed photos. Skips ids that aren't in the
discard pile so it can never bypass the soft-delete safety net.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/discard.py

@@ -3,7 +3,7 @@ Discard API router
 """
 import os
 import logging
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Body
 from sqlalchemy import select, and_
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -47,7 +47,33 @@ async def empty_discard(db: AsyncSession = Depends(get_db)):
         select(Photo).where(Photo.is_discarded == True)
     )
     photos = result.scalars().all()
+    return await _permanently_delete(db, photos)
 
+
+@router.delete("")
+async def delete_discarded(
+    photo_ids: list[str] = Body(..., embed=True),
+    db: AsyncSession = Depends(get_db),
+):
+    """Permanently delete a specific subset of discarded photos. The photos
+    must already be in the discard pile — non-discarded ids are skipped so
+    this can never bypass the soft-delete safety net.
+    """
+    if not photo_ids:
+        return {"status": "success", "deleted": 0, "file_errors": 0}
+    result = await db.execute(
+        select(Photo).where(
+            and_(Photo.id.in_(photo_ids), Photo.is_discarded == True)
+        )
+    )
+    photos = result.scalars().all()
+    return await _permanently_delete(db, photos)
+
+
+async def _permanently_delete(db: AsyncSession, photos: list[Photo]) -> dict:
+    """Shared helper: unlink files for the given photos and delete their
+    rows. Per-file errors are counted but don't abort the batch.
+    """
     deleted = 0
     file_errors = 0
     for photo in photos: