Cache SEP-10 JWT per anchor to avoid redundant auth round-trips

[Haroldwonder]

Every operation triggers a new SEP-10 challenge/response cycle. Cache the JWT (keyed by anchor domain) and reuse it until it is within threshold_secs of expiry.

[blurbeast]

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

hello my Boss, can i work on this please ??

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

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