Backend: Extend API contract snapshots to vault and transaction list endpoints

[Junirezz]

Context

apiContractSnapshots.ts guards backward compatibility for critical public endpoints (Issue #711). Snapshots currently cover only GET /health and GET /ready.

Problem / Gap

High-traffic vault and transaction list endpoints can break mobile and frontend clients silently. CI does not detect response-shape regressions on /api/v1/vault/* or transaction listing routes.

Proposed approach

• Add Zod schemas and committed JSON snapshots for vault summary and paginated transaction list responses.
• Extend backend/scripts/check-schema-snapshots.ts and CI workflow to validate new endpoints.
• Document the snapshot update workflow in backend README or governance docs.

Acceptance criteria

• At least vault summary and transaction list endpoints have committed schema snapshots.
• CI fails when a required field is removed or changes type.
• Snapshot check script documents how to intentionally bump snapshots.
• OpenAPI definitions stay aligned with snapshot schemas.

Files/areas affected

• backend/src/apiContractSnapshots.ts
• backend/schema-snapshots/
• backend/scripts/check-schema-snapshots.ts
• .github/workflows/backend-governance.yml

[Kingvic300]

@Kingvic300 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would love to work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kingvic300 to this issue.

[boalambo]

@boalambo has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @boalambo to this issue.