Add idempotency-key support to the config write endpoints

[mikewheeleer]

Idempotent config

Description

Retried config writes can double-apply. This issue adds idempotency-key support.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-backend only.
• Accept an Idempotency-Key on config writes and return the stored result on replay within a TTL.
• Reject a reused key with a different body (409).
• Cover first-write, replay, and conflict in tests.

Suggested execution

• Fork the repo and create a branch
• git checkout -b feature/config-91-idem
• Implement changes
  • Write code in: the relevant module.
  • Write comprehensive tests in: cover the new behaviour and edge cases.
• Test and commit

Test and commit

• Run npm run lint, npm test, and npm run build.
• Cover edge cases: first write, replay same, replay different body 409.
• Include the full test output in the PR description.

Example commit message

feat(config): add idempotency keys

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the StableRoute community on Discord: https://discord.gg/37aCpusvx
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — a 5-star rating is much appreciated.