feat(backend): add JWT authentication, RBAC and consent/audit services

Add signup/signin with backend-issued JWT (Argon2 passwords in SurrealDB), role-based authorization policies, per-user data access levels, consent register and read audit log. Protect user endpoints and record sensitive reads.

backend/Application/Abstractions/ITokenService.cs

@@ -0,0 +1,10 @@
+using TarninaHub.Application.Models;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface ITokenService
+{
+    TokenResult CreateToken(UserAccount user);
+}
+
+public sealed record TokenResult(string AccessToken, DateTimeOffset ExpiresAt);