Add a deprecation header on legacy /v1 endpoints with sunset date

[greatest0fallt1me]

Description

This is a backend issue for the GrantFox campaign. Stamp Deprecation and Sunset headers on /v1 endpoints with the planned removal date.

Requirements and Context

• Deprecation header on every /v1 response
• Sunset header with ISO date
• Logged warning on use
• Sunset documented in CHANGELOG
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/deprecation-headers

2. Implement changes
   • src/middleware/deprecation.ts (new)
   • src/routes/index.ts
   • CHANGELOG.md
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: stamp Deprecation/Sunset headers on /v1

Acceptance Criteria

• Headers present
• Logged warning
• CHANGELOG updated
• Tests assert headers

Guidelines

• Minimum 90% test coverage on changed lines
• Input validation at the boundary; standardized error envelope
• Structured logging with correlation IDs
• Clear documentation and inline comments
• Timeframe: 96 hours

[TheShnider]

@TheShnider has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi. I have reviewed this issue and would like to work on it.

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[drips-wave]

Congratulations, @TheShnider! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @TheShnider: 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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[drips-wave]

This issue has been completed by @TheShnider as part of the Stellar Wave Program's 6th Wave 🥳

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