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/routes/evotor_webhooks.py

@@ -134,8 +134,8 @@ async def user_create(request: Request, db: Session = Depends(get_db)):
         user = User(
             first_name=first_name or "",
             last_name=last_name or "",
-            email=email or f"{evotor_user_id}@evotor.placeholder",
-            phone=phone or "",
+            email=email or f"{evotor_user_id}@evotor.invalid",
+            phone=phone or None,
             password_hash=None,
             role=UserRoleEnum.user,
             status=UserStatusEnum.pending,