Add auth controller/DTOs and update seed password

Introduce authentication API: add AuthController with login, logout and me endpoints using SignInManager/UserManager; add LoginRequest and CurrentUserResponse DTOs. Login enforces active users, updates UpdatedAt on success, and returns localized messages. Also change default seed admin password from "Hoard" to "HoardPassword".

API/Contracts/Auth/CurrentUserResponse.cs

@@ -0,0 +1,11 @@
+namespace API.Contracts.Auth
+{
+    public class CurrentUserResponse
+    {
+        public Guid Id { get; set; }
+        public string UserName { get; set; } = string.Empty;
+        public bool IsAdmin { get; set; }
+        public bool IsActive { get; set; }
+        public bool MustChangePassword { get; set; }
+    }
+}

API/Contracts/Auth/LoginRequest.cs

@@ -0,0 +1,8 @@
+namespace API.Contracts.Auth
+{
+    public class LoginRequest
+    {
+        public string UserName { get; set; } = string.Empty;
+        public string Password { get; set; } = string.Empty;
+    }
+}