feat(planning): add GET /v1/week-plans/{planId}/variety-preview endpoint

Returns currentScore, projectedScore, and scoreDelta when a recipe
would be added on a given date. Used by C6 desktop day picker.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/test/java/com/recipeapp/planning/WeekPlanControllerTest.java

@@ -192,6 +192,25 @@ class WeekPlanControllerTest {
                 .andExpect(jsonPath("$.score").value(7.5));
     }
 
+    @Test
+    void getVarietyPreviewShouldReturn200() throws Exception {
+        var recipeId = UUID.randomUUID();
+        var response = new VarietyPreviewResponse(8.0, 9.0, 1.0);
+
+        when(householdResolver.resolve("sarah@example.com")).thenReturn(HOUSEHOLD_ID);
+        when(planningService.getVarietyPreview(HOUSEHOLD_ID, PLAN_ID, recipeId, WEEK_START.plusDays(2)))
+                .thenReturn(response);
+
+        mockMvc.perform(get("/v1/week-plans/{planId}/variety-preview", PLAN_ID)
+                        .principal(() -> "sarah@example.com")
+                        .param("recipeId", recipeId.toString())
+                        .param("date", "2026-04-08"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.currentScore").value(8.0))
+                .andExpect(jsonPath("$.projectedScore").value(9.0))
+                .andExpect(jsonPath("$.scoreDelta").value(1.0));
+    }
+
     @Test
     void addSlotShouldReturn403ForMemberRole() throws Exception {
         SecurityContextHolder.getContext().setAuthentication(