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

feat(emailer): add Postmark and Resend providers

backend/Modules/Tipping/Contracts/ITipPaymentNotifier.cs

@@ -0,0 +1,6 @@
+namespace Hutopy.Modules.Tipping.Contracts;
+
+public interface ITipPaymentNotifier
+{
+    Task NotifyPaymentSucceedAsync(string stripeId, string invoiceUrl, CancellationToken ct);
+}

backend/Modules/Tipping/Contracts/ITipProcessor.cs

@@ -0,0 +1,16 @@
+using Hutopy.Modules.Creators.Contracts;
+
+namespace Hutopy.Modules.Tipping.Contracts;
+
+public interface ITipProcessor
+{
+    Task<TipCheckoutSession> CreateCheckoutSessionAsync(
+        Guid tipId,
+        CreatorReference creator,
+        decimal amount,
+        string currency,
+        string message,
+        string successUrl,
+        string cancelUrl,
+        CancellationToken ct = default);
+}

backend/Modules/Tipping/Contracts/TipCheckoutSession.cs

@@ -0,0 +1,5 @@
+namespace Hutopy.Modules.Tipping.Contracts;
+
+public record TipCheckoutSession(
+    string Id,
+    string Url);