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>

frontend/src/components/timeline/PhotoThumbnail.tsx

@@ -140,7 +140,10 @@ export function PhotoThumbnail({
             alt={photo.filename}
             className={clsx(
               'h-full w-full object-cover transition-opacity duration-200',
-              imageLoaded ? 'opacity-100' : 'opacity-0'
+              imageLoaded ? 'opacity-100' : 'opacity-0',
+              // Discarded photos fade out + desaturate so the trash section
+              // reads as a trash section, not just another grid view.
+              photo.is_discarded && 'opacity-50 grayscale'
             )}
             onLoad={handleImageLoad}
             onError={handleImageError}
@@ -216,7 +219,12 @@ export function PhotoThumbnail({
           </div>
         )}
         {photo.is_discarded && (
-          <Trash2 className="h-4 w-4 text-reject" />
+          <div
+            className="flex h-5 w-5 items-center justify-center rounded-full bg-reject text-white shadow-md"
+            title="Discarded"
+          >
+            <Trash2 className="h-3 w-3" />
+          </div>
         )}
       </div>