feat: add guest login button for customer demos

- One-click "访客体验入口" button on login page
- Visitor account (visitor/visitor123) with read-only role
- No username/password needed for customers to browse

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/seed_data.py

@@ -108,6 +108,9 @@ async def seed():
             User(username="operator1", hashed_password=hash_password("tianpu123"),
                  full_name="李运维", role="operator", email="op1@tianpu.com",
                  phone="13800000003"),
+            User(username="visitor", hashed_password=hash_password("visitor123"),
+                 full_name="访客", role="visitor", email=None,
+                 phone=None),
         ]
         session.add_all(users)