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/admin.py

@@ -101,6 +101,30 @@ async def admin_user_detail(user_id: int, request: Request, db: Session = Depend
     return _render(request, "admin/user_detail.html", {"user": admin, "target": target})
 
 
+# ── View-as ───────────────────────────────────────────────────────────────────
+
+@router.post("/users/{user_id}/view-as")
+async def admin_view_as(user_id: int, request: Request, db: Session = Depends(get_db)):
+    try:
+        _admin_user(request, db)
+    except Exception:
+        return RedirectResponse("/login", 303)
+    target = db.get(User, user_id)
+    if target:
+        request.session["viewed_user_id"] = user_id
+    return RedirectResponse("/connections", 303)
+
+
+@router.post("/view-as/stop")
+async def admin_view_as_stop(request: Request, db: Session = Depends(get_db)):
+    try:
+        _admin_user(request, db)
+    except Exception:
+        return RedirectResponse("/login", 303)
+    request.session.pop("viewed_user_id", None)
+    return RedirectResponse("/admin/users", 303)
+
+
 # ── User actions ──────────────────────────────────────────────────────────────
 
 @router.post("/users/{user_id}/activate")