Add public response schema version header

[Chucks1093]

Summary

Add an optional response header that exposes a public API schema/version identifier.

Acceptance Criteria

• header value comes from config/constants
• can be enabled/disabled per environment
• does not change existing response bodies

Contributor coordination: https://t.me/accesslayerorg

[Wilfred007]

@Wilfred007 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

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Congratulations, @Wilfred007! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

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[drips-wave]

This issue has been completed by @Wilfred007 as part of the Stellar Wave Program's 3rd Wave 🥳

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