Backend: add token expiry (expiresIn/expiresAt) to /auth/verify response

[bbjiggy]

Description

Follow-up from the #79 spike (see docs/spikes/issue-79-retry-session-multisig.md).

auth/challenge.ts's verifyAndGetToken exchanges a signature for a session token via the
backend's /auth/verify endpoint, but the response currently only returns { token } — no
expiry information. The SDK's session layer (src/auth/session.ts) now supports storing an
expiresAt alongside the token and exposes isSessionExpired(), but without a backend-supplied
TTL it can only apply a conservative client-side default (currently 15 minutes), which is a guess,
not a guarantee of the token's real lifetime.

Proposal

Extend the /auth/verify response with expiresIn (seconds) or expiresAt (UNIX ms), and update
verifyAndGetToken to pass it through to saveSession.

Acceptance Criteria

• Backend /auth/verify response includes a token expiry field
• verifyAndGetToken in the SDK reads it and forwards it to saveSession
• Fallback to the existing client-side default remains in place for backwards compatibility with older backend deployments

[Buluz244]

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I love to be assigned this issue also I've experienced across many framework and I'm confidently able to resolve it looking forward to it thanks

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[Securify001]

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Hi. I am a web3 fullstack developer. Kindly assign this issue to me

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[oluwagbemigaphilip]

@oluwagbemigaphilip has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Please kindly assign me, will do a perfect job to this

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