fix(security): make rate-limit failure policy configurable and fail-closed for auth/OTP endpoints

[cypriannwokolo2-creator]

Summary

internal/api/middleware/ratelimit.go:35-37 fails open (falls back to in-memory limiter) on Redis outage. For auth/OTP/swap endpoints this weakens DoS and brute-force protection exactly when the shared limiter is down.

Acceptance Criteria

• Per-route failure policy (fail-closed for sensitive routes)
• Fail-open only for low-risk GET routes
• Tests for Redis-down behavior per policy

Complexity

High.

[danfulani0]

@danfulani0 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer! I'm a senior developer with experience building high-quality, production-ready software. I'd love to take on this issue and deliver a clean, well-tested solution that aligns with the project's standards. Please assign it to me—I'm ready to get started and contribute effectively.

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[EDOHWARES]

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

GM GM Maintainer, pls can I work on this issue ? ETA - 12hrs

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