feat(security): rate limit by wallet address in addition to IP (#449)

[abdulqoyumadegoke]

Closes #449

Summary

Adds wallet address keyed rate limiting in addition to existing IP-based rate limiting across endpoints to prevent distributed abusive request patterns against single wallets or shared NAT IP collision blocks.

Changes Made

• Added \checkWalletRateLimit\ and \checkDualRateLimit\ helpers in \src/lib/observability/rateLimiter.ts.
• Integrated wallet-based rate limiting on /api/auth/challenge\ when wallet addresses are provided.
• Added comprehensive unit tests covering wallet address rate limiting in \�pi/auth/challenge.test.ts\ and \src/test/observability.test.ts.

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