feat: remove register, add evo webhooks, admin view-as user

- Remove /register route and nav links (users created via Evotor webhook)
- Fix evotor_webhooks.py: use phone=None instead of phone="" to avoid unique constraint
- Add admin "view as user" feature: POST /admin/users/{id}/view-as sets viewed_user_id
  in session; POST /admin/view-as/stop clears it
- catalog, vk_catalog, sync, connections GET routes use get_viewed_user() so admins
  see another user's data while browsing
- Orange banner shown at top when admin is viewing as another user

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/admin/user_detail.html

@@ -96,6 +96,11 @@
                         <i class="bi bi-envelope me-1"></i>Отправить приглашение
                     </button>
                 </form>
+                <form method="post" action="/admin/users/{{ target.id }}/view-as">
+                    <button type="submit" class="w-100 outline">
+                        <i class="bi bi-eye me-1"></i>Просмотр от имени пользователя
+                    </button>
+                </form>
                 {% if user.role == 'system' and target.id != user.id %}
                 <form method="post" action="/admin/users/{{ target.id }}/delete"
                       onsubmit="return confirm('Удалить пользователя {{ target.email }}? Это действие необратимо.')">