Task/markets etag v7

[pchieneye]

feat: add ETag support for /api/markets

Summary

This PR adds ETag support for the public markets endpoints so clients can perform conditional GET requests and receive 304 Not Modified responses when the response payload has not changed.

What changed

• Added strong ETag generation and conditional revalidation handling for the public markets list and market detail responses.
• Returned ETag and Cache-Control: no-cache headers for the relevant GET responses.
• Added focused regression tests covering ETag headers and conditional revalidation behavior for the markets listing endpoint.
• Documented the new caching/conditional request behavior in the markets API docs.

Closes: #387

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[greatest0fallt1me]

Well-scoped PR — merging. Thanks for the contribution!