feat: add middle_name field and map all three Evotor name fields

- Add middle_name column to users table (migration 0012)
- Map Evotor's second_name → last_name, middle_name → middle_name
  in /user/create webhook handler
- Update name fields on existing users when Evotor sends them

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

web/models/user.py

@@ -23,6 +23,7 @@ class User(Base):
     id = Column(Integer, primary_key=True, autoincrement=True)
     first_name = Column(String(100), nullable=False)
     last_name = Column(String(100), nullable=False)
+    middle_name = Column(String(100), nullable=True)
     email = Column(String(255), nullable=False)
     phone = Column(String(20), nullable=True)
     password_hash = Column(String(255), nullable=True)