Update a fixed threat-model section

Update the markdown content of one of the fixed threat-model sections (Components, Interfaces, Actor Types, Authn/Authz Model, Control Plane, Attacker Types, Application-Specific Attacker Objectives, Datastores, Assumptions, Out of Scope). Section identity is the section row's id — call threatModel.getByApplication first to find it. The content is rendered inside the <threat_model> XML block of investigation, validation, and triage prompts on the next scan, where it gives agents the app-specific security baseline they should reason from. Section type cannot be changed — to change which slot you edit, choose a different sectionId. Requires ORG_EDIT_SAST_CONFIG.