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

feat(emailer): add Postmark and Resend providers

backend/Modules/Identity/Contracts/IUserLookup.cs

@@ -0,0 +1,6 @@
+namespace Hutopy.Modules.Identity.Contracts;
+
+public interface IUserLookup
+{
+    Task<UserReference?> GetUserAsync(Guid userId, CancellationToken cancellationToken = default);
+}

backend/Modules/Identity/Contracts/KnownRoles.cs

@@ -0,0 +1,7 @@
+namespace Hutopy.Modules.Identity.Contracts;
+
+public static class KnownRoles
+{
+    public const string Administrator = nameof(Administrator);
+    public const string Creator = nameof(Creator);
+}

backend/Modules/Identity/Contracts/UserReference.cs

@@ -0,0 +1,6 @@
+namespace Hutopy.Modules.Identity.Contracts;
+
+public record UserReference(
+    Guid Id,
+    string Fullname,
+    string? PortraitUrl);