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'

backend/app/api/ingredients.py

@@ -57,9 +57,24 @@ def get_ingredient(ingredient_id: UUID, db: Session = Depends(get_db)) -> Ingred
 
 @public_router.post("", response_model=IngredientRead, status_code=status.HTTP_201_CREATED)
 def create_ingredient_public(payload: IngredientCreate, db: Session = Depends(get_db)) -> Ingredient:
+    name_lower = payload.name.lower()
+    # idempotent: return existing if name or alias already known
+    existing = (
+        db.query(Ingredient)
+        .filter(
+            or_(
+                Ingredient.name_lower == name_lower,
+                Ingredient.aliases.any(payload.name.lower()),
+            )
+        )
+        .first()
+    )
+    if existing:
+        return existing
+
     row = Ingredient(
         name=payload.name,
-        name_lower=payload.name.lower(),
+        name_lower=name_lower,
         aliases=payload.aliases,
         aisle=payload.aisle,
         unit=payload.unit,
@@ -70,6 +85,19 @@ def create_ingredient_public(payload: IngredientCreate, db: Session = Depends(ge
         db.commit()
     except IntegrityError:
         db.rollback()
+        # race-window safety: re-check after rollback
+        existing = (
+            db.query(Ingredient)
+            .filter(
+                or_(
+                    Ingredient.name_lower == name_lower,
+                    Ingredient.aliases.any(payload.name.lower()),
+                )
+            )
+            .first()
+        )
+        if existing:
+            return existing
         raise HTTPException(status_code=409, detail="ingredient name already exists")
     db.refresh(row)
     return row