feat(stability): unify readiness and recovery

backend/Controllers/AdminController.cs

@@ -2,6 +2,7 @@ using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Nexus.Api.Data;
 using Nexus.Api.DTOs;
+using Nexus.Api.Http;
 using Nexus.Api.Repositories;
 using Nexus.Api.Services;
 
@@ -74,7 +75,9 @@ public class AdminController(
         var normalizedEmail = AuthService.NormalizeEmail(request.Email);
         var existing = await userRepository.GetByEmailAsync(normalizedEmail, ct);
         if (existing is not null)
-            return Results.Conflict(new { error = "A user with this email already exists." });
+            return NexusHttpResults.Problem(
+                StatusCodes.Status409Conflict,
+                "A user with this email already exists.");
 
         var user = new NexusUser
         {
@@ -108,7 +111,9 @@ public class AdminController(
     {
         var user = await userRepository.GetByIdAsync(id, ct);
         if (user is null)
-            return Results.NotFound(new { error = "User not found." });
+            return NexusHttpResults.Problem(
+                StatusCodes.Status404NotFound,
+                "User not found.");
 
         if (string.Equals(user.Role, "owner", StringComparison.OrdinalIgnoreCase))
             return Results.Problem("Owner accounts cannot be deleted via API.", statusCode: 403);
@@ -142,7 +147,9 @@ public class AdminController(
 
         var user = await userRepository.GetByIdAsync(id, ct);
         if (user is null)
-            return Results.NotFound(new { error = "User not found." });
+            return NexusHttpResults.Problem(
+                StatusCodes.Status404NotFound,
+                "User not found.");
 
         // Niemals owner überschreiben
         if (string.Equals(user.Role, "owner", StringComparison.OrdinalIgnoreCase))