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.