api+admin: let the curator take a photo off the landing, not just move it

Uploads still land in the community strip with consent on, by the uploader's own
tick — that default stays. What was missing is the curator's removal: the slot
select only offered the other three live placements, so "off the strip" meant
publishing the photo somewhere else or deleting the uploader's row.

`off` is a fifth slot value. The reel draws slot === 'strip' and each live slot
draws its own, so an `off` photo renders nowhere on the landing, while its owner
still has it in MY PHOTOS.

docker/frontend/src/Admin.tsx

@@ -19,13 +19,15 @@ import type { MsgKey } from './i18n/vi';
 type State = 'loading' | 'guest' | 'forbidden' | 'ready';
 type Tab = 'profile' | 'users' | 'pictures' | 'stats';
 
-// The four places a photo can be drawn from. `strip` feeds the community reel;
-// each live slot shows one photo of its set, picked at random per page load.
+// Where a photo is drawn from. `strip` feeds the community reel and each live
+// slot shows one photo of its set, picked at random per page load. `off` is the
+// way off the landing without deleting the uploader's row.
 const SLOTS: { id: PhotoSlot; key: MsgKey }[] = [
   { id: 'strip', key: 'adm.slotStrip' },
   { id: 'tester', key: 'adm.slotTester' },
   { id: 'creator', key: 'adm.slotCreator' },
   { id: 'qr', key: 'adm.slotQr' },
+  { id: 'off', key: 'adm.slotOff' },
 ];
 
 // The left column, in the order it reads.