feat(auth): add admin bootstrap and username login

Initialize admin bootstrap settings during startup, persist username support in auth flows, and align frontend auth requests with local API behavior.

backend/app/models/user.py

@@ -5,6 +5,7 @@ from app.models.base import BaseModel
 class User(BaseModel):
     __tablename__ = "users"
 
+    username = Column(String(255), unique=True, nullable=False, index=True)
     email = Column(String(255), unique=True, nullable=False, index=True)
     hashed_password = Column(String(255), nullable=False)
     full_name = Column(String(255), nullable=True)