Add migration preflight checks for invoice schema changes

[mikewheeleer]

Summary

Schema changes can start with incompatible data and fail halfway through a deployment.

Why this matters

This closes a correctness, security, or operability gap in the Liquifact backend and makes the behavior explicit, observable, and safe under failure.

Requirements

• Repository scope: Liquifact/Liquifact-backend only.
• Inspect counts and invariants before migration; fail without writes when preconditions are not met; emit a redacted report suitable for operators.
• Preserve tenant isolation, structured errors, and existing API compatibility unless the issue explicitly requires a contract change.
• Add focused unit and integration coverage for the new behavior.

Technical guidance

• Follow the repository's existing middleware, service, persistence, logging, and error-envelope conventions.
• Keep side effects bounded and make retries, authorization, and failure handling explicit.
• Document non-obvious tradeoffs and include security notes in the PR.

Edge cases — each must have a test

• empty database
• invalid legacy row
• partial prior migration
• very large table
• preflight repeated

Acceptance criteria

• Every requirement and edge case above is implemented and tested
• Unit and integration tests cover success, failure, retry, and authorization paths
• Existing tests pass with no regressions
• Errors are structured and do not leak secrets or internal stack details
• npm run lint, npm test, and npm run build passes locally
• Code follows existing conventions with no duplication or dead code
• Documentation or concise comments explain non-obvious decisions
• PR description explains the approach, includes test evidence and security notes, and references Closes #

Out of scope

• automatic destructive cleanup

[supremeproton01]

@supremeproton01 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer, can i please be assigned to this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @supremeproton01 to this issue.

[drips-wave]

Congratulations, @supremeproton01! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @supremeproton01: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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