feat: Privy server SDK evaluation and base configuration (#593, #594, #595, #596)

[oomokaro1]

Closes #593, Closes #594, Closes #595, Closes #596

Type of Change

• Documentation update
• New feature (non-breaking change that adds functionality)

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Summary

Adds a Privy server SDK evaluation spike document (#593) formalizing the decision to use native crypto + fetch instead of @privy-io/node. Issues #594–#596 (SDK dependency, env config, client singleton) were already implemented in the codebase — this PR links them and documents the rationale.

Motivation / Context

Part of the Privy Authentication (Backend) initiative. The implementation already exists across config modules (src/config/privy.ts, src/config/privyAuth.ts), environment validation (src/config/env.ts), and the service layer (src/services/PrivyService.ts, src/services/privy/). This PR adds the missing research spike doc to close out the evaluation task and links all four issues.

Closes #593, Closes #594, Closes #595, Closes #596

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