Backend: Add API versioning via header

[Smartdevs17]

Problem

URL-based versioning only.

Proposed Solution

Add header versioning.

Acceptance Criteria

• Accept version header
• Support Content-Type versioning
• Maintain backward compat
• Document versions

Technical Notes

• Files: backend/src/index.ts

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[Adedayo-Data]

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

Hi Maintiainer
I would love to work on this issue. I have read the task requirement and would submit a PR before the wave is over

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