Sunset header and feature-flagged retirement for legacy routes

[kilodesodiq-arch]

Problem Statement. Legacy /ai/* paths exist alongside /v1/ai/*. The webhook in
swagger.config.ts says "deprecated" but no Sunset header is emitted.

Why it matters. Consumers (the mobile + frontend) get no machine-readable signal to
migrate.

Technical Context. RFC 8594 Sunset and Deprecation headers.

Expected Outcome.

• Legacy handlers emit Sunset: <RFC 1123 date> (e.g. Sunset: Wed, 01 Oct 2026 00:00:00 GMT) and Deprecation: true.
• A configurable LEGACY_RETIREMENT_DATE env renders the header.

Acceptance Criteria.

• A test asserts both headers on every /ai/* route.
• Backwards compatibility preserved (still 308s to /v1/ai/*).

Files or modules likely to be affected.
app/ai-service/main.py.

Difficulty. Easy
Estimated effort. XS

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Backlog item #58 from `docs/maintainer-issue-backlog.md.

[AugistineCreates]

I would like to work on this issue because i am confident i can resolve the issue, having worked on and resolved similar issues. my eta is 4 hours

[grantfox-oss]

🦊 GrantFox — @AugistineCreates has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #266)
2. Your PR will be reviewed by the ChainForgee maintainers

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@AugistineCreates's PR #331 was approved and merged by @kilodesodiq-arch.

🏆 @AugistineCreates: You earned 40 FoxPoints for this contribution! Your current tier: Builder (2,914 total points). Track your full progress on GrantFox.

👏 Great work, @AugistineCreates! Keep contributing to ChainForgee.