fix(pantry): make public ingredient endpoint idempotent

Backend:
- POST /api/ingredients now checks name_lower and aliases before inserting
- Returns existing ingredient on 409 instead of throwing error

Frontend:
- Removed fragile 409-recovery logic from Pantry.tsx handleAdd
- Added aliases field to Ingredient type for case-insensitive matching

Fixes pantry add for ingredients like 'Carrots' whose canonical name is 'Carrot'

frontend/src/pages/Pantry.tsx

@@ -31,10 +31,15 @@ export default function Pantry() {
     queryFn: () => mealPlannerApi.recipes.listIngredients().then(r => r.data),
   })
 
+  function _ingredientMatchesName(ing: Ingredient, search: string): boolean {
+    const s = search.toLowerCase()
+    return ing.name.toLowerCase() === s || (ing.aliases ?? []).some(a => a.toLowerCase() === s)
+  }
+
   /* fuzzy match existing ingredient */
   const matchedIngredient = ingredientName.trim().length > 0
     ? ingredients?.find(
-        ing => ing.name.toLowerCase() === ingredientName.trim().toLowerCase()
+        ing => _ingredientMatchesName(ing, ingredientName.trim())
       )
     : undefined
 
@@ -86,12 +91,8 @@ export default function Pantry() {
         ingredientId = created?.id
         // refresh ingredient list so next add sees it
         await queryClient.invalidateQueries({ queryKey: ['ingredients'] })
-      } catch (err: any) {
-        if (err?.response?.status === 409) {
-          showToast.error('Ingredient name already exists')
-        } else {
-          showToast.error('Failed to create ingredient')
-        }
+      } catch {
+        showToast.error('Failed to create ingredient')
         return
       }
     }