refactor(core): move identity models from API to Core

src/FiscalOS.Core/Identity/User.cs

@@ -0,0 +1,9 @@
+namespace FiscalOS.Core.Identity;
+
+public sealed class User : Entity
+{
+  public string Username { get; init; } = string.Empty;
+  public string HashedPassword { get; init; } = string.Empty;
+
+  public ICollection<RefreshToken> RefreshTokens { get; init; } = [];
+}