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/login.html

@@ -17,8 +17,7 @@
                     <button type="submit" class="w-100">Войти</button>
                 </form>
                 <div class="text-center small mt-3">
-                    <a href="/forgot-password">Забыли пароль?</a><br>
-                    <a href="/register">Зарегистрироваться</a>
+                    <a href="/forgot-password">Забыли пароль?</a>
                 </div>
             </div>
         </article>