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);

assets/js/app.js

@@ -3231,6 +3231,9 @@ function showAddForm() {
         vacuumCb.checked = false;
         document.getElementById('add-vacuum-hint').style.display = 'none';
     }
+    // Reset historical expiry for this product; will be fetched async
+    window._historyExpiryDays = null;
+    window._historyExpiryCount = 0;
     // Store base expiry for vacuum recalculation
     window._addBaseExpiryDays = estimatedDays;
     
@@ -3258,6 +3261,10 @@ function showAddForm() {
     `;
     
     showPage('add');
+    // After rendering, fetch history-based expiry prediction
+    if (currentProduct && currentProduct.id) {
+        _fetchExpiryHistoryAndUpdate(currentProduct.id);
+    }
 }
 
 function toggleVacuumSealed() {
@@ -3277,16 +3284,17 @@ function recalculateAddExpiry() {
     const loc = document.getElementById('add-location')?.value || '';
     const isVacuum = document.getElementById('add-vacuum-sealed')?.checked;
     
-    let days = estimateExpiryDays(currentProduct, loc);
-    if (isVacuum) days = getVacuumExpiryDays(days);
+    const baseDays = window._historyExpiryDays ?? estimateExpiryDays(currentProduct, loc);
+    let days = isVacuum ? getVacuumExpiryDays(baseDays) : baseDays;
     
-    window._addBaseExpiryDays = estimateExpiryDays(currentProduct, loc);
+    window._addBaseExpiryDays = baseDays;
     
     const newDate = addDays(days);
     const newLabel = formatEstimatedExpiry(days);
     
     let suffix = '';
-    if (loc === 'freezer' && isVacuum) suffix = ' (freezer + sotto vuoto)';
+    if (window._historyExpiryDays) suffix = ' (da storico)';
+    else if (loc === 'freezer' && isVacuum) suffix = ' (freezer + sotto vuoto)';
     else if (loc === 'freezer') suffix = ' (freezer)';
     else if (isVacuum) suffix = ' (sotto vuoto)';
     
@@ -3298,6 +3306,33 @@ function recalculateAddExpiry() {
     if (dateEl) dateEl.textContent = formatDate(newDate);
 }
 
+async function _fetchExpiryHistoryAndUpdate(productId) {
+    try {
+        const res = await fetch(`api/index.php?action=expiry_history&product_id=${encodeURIComponent(productId)}`);
+        const data = await res.json();
+        if (data.avg_days && data.avg_days > 0 && data.count >= 1) {
+            window._historyExpiryDays = data.avg_days;
+            window._historyExpiryCount = data.count;
+            // Update the displayed date and label
+            const loc = document.getElementById('add-location')?.value || '';
+            const isVacuum = document.getElementById('add-vacuum-sealed')?.checked;
+            let days = isVacuum ? getVacuumExpiryDays(data.avg_days) : data.avg_days;
+            const newDate = addDays(days);
+            const newLabel = formatEstimatedExpiry(days);
+            const suffix = ` <span class="history-badge" title="Media da ${data.count} insertiment${data.count === 1 ? 'o' : 'i'} precedent${data.count === 1 ? 'e' : 'i'}">📊 storico</span>`;
+            const expiryInput = document.getElementById('add-expiry');
+            const estimateEl = document.querySelector('.expiry-estimate-label');
+            const dateEl = document.querySelector('.expiry-estimate-date');
+            if (expiryInput) expiryInput.value = newDate;
+            if (estimateEl) estimateEl.innerHTML = `Scadenza stimata: <strong>${newLabel}${suffix}</strong>`;
+            if (dateEl) dateEl.textContent = formatDate(newDate);
+            window._addBaseExpiryDays = data.avg_days;
+        }
+    } catch (e) {
+        // silently fall back to rule-based estimate
+    }
+}
+
 function getVacuumExpiryDays(baseDays) {
     // Vacuum sealing extends shelf life significantly
     if (baseDays <= 7) return Math.round(baseDays * 3);       // very fresh: 3x (e.g., 3→9, 7→21)
@@ -3392,12 +3427,13 @@ function selectPurchaseType(btn, type) {
         // Recalculate fresh expiry based on current location/vacuum
         const loc = document.getElementById('add-location')?.value || '';
         const isVacuum = document.getElementById('add-vacuum-sealed')?.checked;
-        let days = estimateExpiryDays(currentProduct, loc);
-        if (isVacuum) days = getVacuumExpiryDays(days);
+        const baseDays = window._historyExpiryDays ?? estimateExpiryDays(currentProduct, loc);
+        let days = isVacuum ? getVacuumExpiryDays(baseDays) : baseDays;
         const estimatedDate = addDays(days);
         const estimateLabel = formatEstimatedExpiry(days);
         let suffix = '';
-        if (loc === 'freezer' && isVacuum) suffix = ' (freezer + sotto vuoto)';
+        if (window._historyExpiryDays) suffix = ` <span class="history-badge" title="Media da ${window._historyExpiryCount} inserimento/i precedente/i">📊 storico</span>`;
+        else if (loc === 'freezer' && isVacuum) suffix = ' (freezer + sotto vuoto)';
         else if (loc === 'freezer') suffix = ' (freezer)';
         else if (isVacuum) suffix = ' (sotto vuoto)';