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/services/api.ts

@@ -305,6 +305,16 @@ export const discard = {
     const response = await api.delete('/discard/empty')
     return response.data
   },
+
+  /** Permanently delete a specific subset of discarded photos. The backend
+   *  silently skips ids that aren't in the pile, so this can never bypass
+   *  the soft-delete safety net. */
+  deletePermanent: async (photoIds: string[]) => {
+    const response = await api.delete('/discard', {
+      data: { photo_ids: photoIds },
+    })
+    return response.data
+  },
 }
 
 export default api