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/Interfaces/IUserPaymentLock.cs

@@ -0,0 +1,32 @@
+namespace CMSMicroservice.Application.Common.Interfaces;
+
+/// <summary>
+/// Per-scope in-process mutex for payment flows (initiate / verify).
+/// Prevents the same user from running duplicate gateway operations concurrently.
+/// </summary>
+public interface IUserPaymentLock
+{
+    Task<T> ExecuteAsync<T>(
+        string scope,
+        PaymentLockStrategy strategy,
+        Func<CancellationToken, Task<T>> action,
+        CancellationToken cancellationToken = default);
+
+    Task ExecuteAsync(
+        string scope,
+        PaymentLockStrategy strategy,
+        Func<CancellationToken, Task> action,
+        CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// How to behave when the payment lock is already held.
+/// </summary>
+public enum PaymentLockStrategy
+{
+    /// <summary>Wait up to the configured timeout (callback / verify paths).</summary>
+    WaitForRelease,
+
+    /// <summary>Reject immediately (initiate / double-click paths).</summary>
+    FailFast
+}