feat(rewards): add 14 new shop items with twin backgrounds

Each page background gets a matching banner background. Tests verify
pairing, price constraints, and code length limits

src/test/java/core_service/modules/rewards/domain/ShopItemTest.java

@@ -0,0 +1,33 @@
+package core_service.modules.rewards.domain;
+
+import core_service.shared.test.AbstractUnitTest;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class ShopItemTest extends AbstractUnitTest {
+
+    @Test
+    void everyPageBackgroundHasABannerTwin() {
+        for (ShopItem item : ShopItem.values()) {
+            if (!item.name().startsWith("BG_")) continue;
+            ShopItem twin = ShopItem.valueOf("BN_" + item.name().substring(3));
+            assertThat(item.slot()).isEqualTo(LookSlot.PAGE_BACKGROUND);
+            assertThat(twin.slot()).isEqualTo(LookSlot.BANNER_BACKGROUND);
+        }
+    }
+
+    @Test
+    void buyableBannerBackgroundsCost300() {
+        assertThat(Arrays.stream(ShopItem.values())
+                .filter(item -> item.name().startsWith("BN_") && !item.unlockOnly())
+                .map(ShopItem::price)).containsOnly(300);
+    }
+
+    @Test
+    void codesFitTheColumn() {
+        assertThat(Arrays.stream(ShopItem.values()).map(Enum::name)).allMatch(name -> name.length() <= 32);
+    }
+}