fix(backend): make transfer lifecycle transitions concurrency-safe

[arisu6804]

Problem

Concurrent cancel, retry, approve, and settle operations can produce impossible states or double settlement.

Objective

Deliver a production-quality improvement to transfer state machine and settlement workers that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define allowed transitions and version checks; make workers idempotent; return safe conflict outcomes to callers.

Acceptance criteria

• Only valid transitions commit; one terminal outcome wins; provider retries cannot settle twice.

Required validation

• State-machine, race, worker-restart, duplicate-callback, and rollback tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.

[tokenjunkielabs]

Claiming #133 as tokenjunkielabs for the GrantFox Third Campaign. I’ll make the transfer lifecycle concurrency-safe end-to-end: explicit allowed transitions, optimistic version checks, one terminal outcome, idempotent provider settlement/retry handling, safe conflict responses, and the required race/restart/duplicate-callback regression coverage. I’ll publish the completed implementation as an actual RemitFlow/RemitFlow-Backend PR.

[guptakumarranjeet150]

Hi maintainers,

I would love to take this on. My planned approach:

• Implement backend logic for fix(backend): make transfer lifecycle transitions concurrency-safe with strict schema validation and idempotent execution.
• Cover route handlers with automated endpoint integration tests.

I am ready to start immediately—please feel free to assign this to me!

[guptakumarranjeet150]

Hello @maintainers,

I would like to resolve this issue. Here is my proposed execution plan:

• Implementation: Structure the service layer for fix(backend): make transfer lifecycle transitions concurrency-safe with secure data access and transaction boundaries.
• Testing: Verify API response schemas, authentication middleware, and database rollback safety.

Excited to work on this—please assign to me and I will submit a clean PR shortly!