feat(payment): add per-user in-memory lock for gateway operations

Introduce IUserPaymentLock to serialize payment initiate and verify flows
per user, preventing concurrent duplicate gateway requests across services.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/CMSMicroservice.Application/Common/PaymentLockScopes.cs

@@ -0,0 +1,19 @@
+namespace CMSMicroservice.Application.Common;
+
+/// <summary>
+/// Canonical scope keys for <see cref="Interfaces.IUserPaymentLock"/>.
+/// </summary>
+public static class PaymentLockScopes
+{
+    /// <summary>One active payment initiation per user (package, wallet charge, order, IPG deposit).</summary>
+    public static string Initiate(long userId) => $"payment:initiate:user:{userId}";
+
+    /// <summary>One active verify per user + authority/order (duplicate callback protection).</summary>
+    public static string Verify(long userId, string resourceKey)
+    {
+        if (string.IsNullOrWhiteSpace(resourceKey))
+            throw new ArgumentException("Payment verify resource key is required.", nameof(resourceKey));
+
+        return $"payment:verify:user:{userId}:{resourceKey.Trim()}";
+    }
+}