feat(products): improve replenishment-aware shopping suggestions

Replace product weight and repurchase intent fields with per-package remaining levels and inventory-first restock signals. Enrich shopping suggestions with usage-aware replenishment scoring so the frontend and LLM can prioritize real gaps and near-empty staples more reliably.

backend/tests/test_inventory.py

@@ -24,12 +24,17 @@ def test_update_inventory_opened(client, created_product):
 
     r2 = client.patch(
         f"/inventory/{inv_id}",
-        json={"is_opened": True, "opened_at": "2026-01-15"},
+        json={
+            "is_opened": True,
+            "opened_at": "2026-01-15",
+            "remaining_level": "low",
+        },
     )
     assert r2.status_code == 200
     data = r2.json()
     assert data["is_opened"] is True
     assert data["opened_at"] == "2026-01-15"
+    assert data["remaining_level"] == "low"
 
 
 def test_update_inventory_not_found(client):