Bandwidth caps for peer replication

## Problem
Catch-up and reactive replication share the worker pool but have no bandwidth limit. A node rejoining with a large backlog can saturate the network.

## Proposed Solution
Add a configurable bytes/sec rate limiter on the puller's fetch path. Per-node or cluster-wide.

## Context
Deferred from Phase 3. Current mitigation: stagger rejoins, tune \`replication_pull_workers\` down.

## Effort
Medium