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/IConsentService.cs

@@ -0,0 +1,20 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface IConsentService
+{
+    Task<ConsentResponse> GrantAsync(
+        string userId,
+        GrantConsentRequest request,
+        CancellationToken cancellationToken);
+
+    Task<IReadOnlyList<ConsentResponse>> ListForUserAsync(
+        string userId,
+        CancellationToken cancellationToken);
+
+    Task<bool> RevokeAsync(
+        string consentId,
+        string userId,
+        CancellationToken cancellationToken);
+}