Webhook-to-SSE gateway with hierarchical topic routing and signature verification

Clean up empty subscriber map entries and add TTL to Redis buffer master

The broker's subscriber map accumulated empty slices for paths that no longer had any subscribers. Now it deletes the key when the last one unsubscribes. The Redis buffer key also had no expiry, so it would stick around forever after all instances shut down — now it gets a 24h TTL refreshed on each publish. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>