fix(backend): make stream state transitions concurrency-safe

[arisu6804]

Problem

Concurrent cancel, pause, top-up, and withdraw requests can observe stale state and apply incompatible transitions.

Objective

Deliver a production-quality improvement to stream lifecycle service and persistence that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Define an explicit state machine and allowed transitions; use optimistic or pessimistic concurrency control; return version conflicts safely.

Acceptance criteria

• Invalid transitions fail atomically; exactly one concurrent transition wins; clients receive the resulting version and state.

Required validation

• State-machine, race, retry, stale-version, and failure-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.

[greatest0fallt1me]

Hello Maintainers 👋,
I’m interested in picking up this issue and contributing to the project.
I have solid experience working across both frontend and blockchain layers, including building user interfaces with Next.js and developing Stellar smart contracts using Rust (Soroban). I’m familiar with translating Figma designs into pixel-accurate UIs, handling wallet integrations, and writing efficient, well-tested contract logic.
I focus on writing clean, maintainable code, following repository standards, and keeping communication transparent throughout the development process. I can start immediately and complete the work within the expected timeframe.
Excited to collaborate and add value to the project 🚀
Thank you!

[grantfox-oss]

🦊 GrantFox — @greatest0fallt1me has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #71)
2. Your PR will be reviewed by the StreamPay-Organization maintainers

Good luck! Track your progress on GrantFox.