Create a candidate interview invitation

Update an existing interview template. Send at least one field. `basePrompt` is visible in responses but is not accepted as input. Sending `description` regenerates `basePrompt`, template sections, and whiteboard tasks synchronously through the internal generation flow. LLM calls time out after 15 seconds, and a request that has already spent 20 seconds before persistence returns a retryable `503` without starting the template update. `whiteboardEnabled` is accepted only together with `description`; when omitted, regeneration keeps the template's persisted whiteboard setting. Discovery templates reject `whiteboardEnabled: true`. Concurrent template edits are serialized; callers may receive a retryable `409` while another template mutation holds the lifecycle fence.