feat(api): add API endpoints for survey response answers

[ericfitz]

Summary

Add REST API endpoints for reading and querying extracted survey response answers from the survey_answers table.

Background

Issue #178 added the internal survey_answers table and SurveyAnswerStore interface, which extracts and stores structured answers on every response save. Currently this data is only accessible internally (e.g., by create_threat_model via GetFieldMappings). External API access would enable:

• Clients to display individual answers with question metadata
• Dashboards to query answers by field mapping
• Integrations to read structured answer data without parsing opaque survey_json + answers blobs

Proposed Endpoints

Method	Path	Description
GET	/intake/survey_responses/{id}/answers	List extracted answers for a response
GET	/intake/survey_responses/{id}/answers/field_mappings	List answers with mapsToTmField annotations, keyed by target field
GET	/triage/survey_responses/{id}/answers	Same as above, for triage context
GET	/triage/survey_responses/{id}/answers/field_mappings	Same as above, for triage context

Requirements

1. Add endpoints to OpenAPI spec (api-schema/tmi-openapi.json)
2. Define response schemas for answer listings
3. Regenerate API code (make generate-api)
4. Implement handlers using existing SurveyAnswerStore interface
5. Respect existing authorization (same access rules as the parent survey response)
6. Add unit and integration tests

Dependencies

• feat(api): extract survey questions into separate table on survey create/update #178 (completed) — survey_answers table and SurveyAnswerStore interface