Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

api/models/account.py

@@ -203,7 +203,7 @@ class Tenant(Base):
     plan: Mapped[str] = mapped_column(db.String(255), server_default=db.text("'basic'::character varying"))
     status: Mapped[str] = mapped_column(db.String(255), server_default=db.text("'normal'::character varying"))
     custom_config: Mapped[Optional[str]] = mapped_column(db.Text)
-    created_at: Mapped[datetime] = mapped_column(db.DateTime, server_default=func.current_timestamp())
+    created_at: Mapped[datetime] = mapped_column(db.DateTime, server_default=func.current_timestamp(), nullable=False)
     updated_at: Mapped[datetime] = mapped_column(db.DateTime, server_default=func.current_timestamp())
 
     def get_accounts(self) -> list[Account]: