feat: predict expiry date from product history when adding items

- PHP: new 'expiry_history' action computes avg shelf life (expiry_date - added_at)
  from inventory table for the same product_id (last 730 days, valid entries only)
- JS: _fetchExpiryHistoryAndUpdate() fires async after showAddForm() renders
  and replaces the rule-based estimate with the historical average if available
- Labeled with '📊 storico' badge on the estimate line (tooltip shows sample count)
- recalculateAddExpiry() and selectPurchaseType('new') both honour window._historyExpiryDays
- Vacuum-sealed multiplier still applied on top of historical base
- Falls back silently to rule-based estimateExpiryDays when no history exists

assets/css/style.css

@@ -2531,6 +2531,17 @@ body {
     font-weight: 600;
 }
 
+.history-badge {
+    font-size: 0.75rem;
+    background: var(--primary);
+    color: #fff;
+    border-radius: 10px;
+    padding: 1px 7px;
+    vertical-align: middle;
+    font-weight: 500;
+    cursor: default;
+}
+
 .form-hint {
     font-size: 0.8rem;
     color: var(--text-muted);