feat(shopping): stats achat produit + édition quantité article

Backend :
- Migration 004 : last_purchased_at (DATE) + avg_interval_days (NUMERIC)
  sur shopping.products
- update_item : met à jour les stats au premier cochage d'un article
  lié à un produit (moyenne mobile exp. 70/30)
- ProductResponse expose les deux nouveaux champs

Frontend :
- ItemRow : long press 500ms → onEdit() (mobile) ; crayon + croix (laptop)
- ShoppingPage : modal édition quantité/unité, état editingItem
- api/shopping.ts : Product inclut last_purchased_at + avg_interval_days

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

backend/app/api/shopping.py

@@ -1,6 +1,7 @@
 # backend/app/api/shopping.py
 import uuid
-from datetime import datetime, timezone
+from datetime import datetime, timezone, date as date_type
+from decimal import Decimal
 from fastapi import APIRouter, Depends, HTTPException, Query
 from fastapi.responses import Response
 from sqlalchemy import select, text, or_
@@ -270,8 +271,28 @@ async def update_item(
     item = result.scalar_one_or_none()
     if not item:
         raise HTTPException(404, "Article introuvable")
+
+    was_checked = item.is_checked
     for field, value in payload.model_dump(exclude_unset=True).items():
         setattr(item, field, value)
+
+    # Mise à jour des stats produit lors du premier cochage
+    if not was_checked and item.is_checked and item.product_id:
+        product = await session.get(Product, item.product_id)
+        if product:
+            today = date_type.today()
+            if product.last_purchased_at and product.last_purchased_at < today:
+                days = (today - product.last_purchased_at).days
+                if product.avg_interval_days is None:
+                    product.avg_interval_days = Decimal(str(days))
+                else:
+                    # Moyenne mobile exponentielle (70 % passé, 30 % nouvel intervalle)
+                    product.avg_interval_days = Decimal(str(
+                        round(float(product.avg_interval_days) * 0.7 + days * 0.3, 1)
+                    ))
+            product.last_purchased_at = today
+            product.frequency_score += 1
+
     await session.commit()
     await session.refresh(item, ["product"])
     return _item_to_response(item)