support PWA notification.

API/ROLAC.API/Services/Notifications/IWebPushSender.cs

@@ -0,0 +1,19 @@
+namespace ROLAC.API.Services.Notifications;
+
+/// <summary>The three fields a browser hands over when it creates a push subscription.</summary>
+public sealed record WebPushTarget(string Endpoint, string P256dh, string Auth);
+
+/// <summary>
+/// Outcome of pushing to one subscription. <see cref="IsExpired"/> is true when the push service
+/// reported the subscription is gone (HTTP 404/410) so the caller can prune it.
+/// </summary>
+public sealed record WebPushSendResult(bool Success, bool IsExpired, string? Error);
+
+/// <summary>
+/// Thin wrapper over the WebPush library's encrypt-and-POST so that <see cref="WebPushService"/>'s
+/// recipient resolution, pruning, and logging can be unit-tested without a real push service.
+/// </summary>
+public interface IWebPushSender
+{
+    Task<WebPushSendResult> SendAsync(WebPushTarget target, string payloadJson, CancellationToken ct = default);
+}