fix: add 2K image default pricing at 1.5x base price

Previously 2K images used the same base price as 1K ($0.134).
Now 2K uses 1.5x multiplier ($0.201), consistent with 4K using 2x ($0.268).

- Backend: add 2K size branch in getDefaultImagePrice
- Frontend: update 2K placeholder from 0.134 to 0.201
- Tests: update assertions for new 2K default price

frontend/src/views/admin/GroupsView.vue

@@ -459,7 +459,7 @@
                 step="0.001"
                 min="0"
                 class="input"
-                placeholder="0.134"
+                placeholder="0.201"
               />
             </div>
             <div>
@@ -1139,7 +1139,7 @@
                 step="0.001"
                 min="0"
                 class="input"
-                placeholder="0.134"
+                placeholder="0.201"
               />
             </div>
             <div>