[GrantFox][High] Define fail-safe rate limiting during Redis outages

[greatest0fallt1me]

Summary

Define fail-safe rate limiting during Redis outages

Why this matters

A Redis outage can silently remove protection or take down every request.

Scope

Implement explicit degraded-mode policy, bounded local fallback, truthful headers, and outage metrics.

Acceptance criteria

• Protected mutation routes fail closed when configured.
• Fallback limits are bounded and documented.
• Recovery returns to distributed limiting without counter corruption.
• Failure-injection tests cover outage, burst, and recovery.

Validation

Add regression coverage for existing behavior, failure modes, authorization boundaries, and compatibility. The implementation must pass the repository CI checks.

Non-goals

• Typo-only, formatting-only, or documentation-only changes.
• Unrelated refactors or dependency upgrades.
• Weakening existing security, authorization, CI, or production safeguards.

Contributor application

Before implementation, comment with relevant experience, a 1–4 bullet approach, and an estimate for opening the first draft PR. Wait for maintainer assignment before coding.

PR requirements

Use a feature branch, include Closes #<issue-number>, check every acceptance criterion, link criteria to code/tests, explain security and failure-mode considerations, and pass CI.

Reward-readiness

This is a substantive GrantFox campaign issue. Merge and CI success do not by themselves guarantee reward eligibility; final reward-readiness is determined by campaign review.