Add Prometheus metrics for federation links (per-peer RTT, packet
counters, active rooms gauge, dedup/rate-limit drop counters).
Add dedup filter (4096-entry ring buffer) to drop duplicate packets
arriving via multiple federation paths. Add per-room token bucket
rate limiter (500 pps) to prevent amplification.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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