fix: wrap updateInventory DB writes in a transaction to prevent concurrent lock errors (#109 #110)

api/index.php

@@ -2252,6 +2252,9 @@ function updateInventory(PDO $db): void {
     $fields[] = "updated_at = CURRENT_TIMESTAMP";
     $params[] = $id;
 
+    // Wrap all writes in a single transaction to avoid concurrent lock failures.
+    $db->beginTransaction();
+    try {
         $stmt = $db->prepare("UPDATE inventory SET " . implode(', ', $fields) . " WHERE id = ?");
         $stmt->execute($params);
 
@@ -2282,7 +2285,13 @@ function updateInventory(PDO $db): void {
             $stmt->execute([$input['package_unit'], $input['package_size'] ?? 0, $input['product_id']]);
         }
 
-    // Real-time Bring! sync: if quantity changed, keep Bring! in sync immediately
+        $db->commit();
+    } catch (Throwable $e) {
+        if ($db->inTransaction()) $db->rollBack();
+        throw $e;
+    }
+
+    // Real-time Bring! sync: done after commit so DB lock is not held during HTTP call
     if (isset($input['quantity']) && $prevRow && abs((float)$input['quantity'] - (float)$prevRow['quantity']) > 0.001) {
         try { bringQuickSyncProduct($db, (int)$prevRow['product_id']); } catch (Throwable $e) {}
     }