many fixes and improvements - rework for modules/ and common/

feat(emailer): add Postmark and Resend providers

backend/Modules/Memberships/Contracts/IMembershipCancellationProcessor.cs

@@ -0,0 +1,6 @@
+namespace Hutopy.Modules.Memberships.Contracts;
+
+public interface IMembershipCancellationProcessor
+{
+    Task<DateTimeOffset?> CancelAsync(string subscriptionId, CancellationToken ct = default);
+}