Move newsletter per-IP limiter to Redis-backed implementation

[hman38705]

Issue 008: Move newsletter per-IP limiter to Redis-backed implementation

Area: Backend
Priority: High
Files: services/api/src/newsletter.rs

Problem

In-memory limiter is per-instance and not safe for horizontal scaling. Multiple instances allow more requests than intended.

Acceptance Criteria

• Limits are shared across all instances via Redis\n- TTL-based counters are atomic\n- Load test confirms consistent behavior under horizontal scale\n- Fallback behavior on Redis failure is defined

[observerr411]

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