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

Smart payload encoding, HTTP method on events, and verification logging master

Text payloads (text/*, form-urlencoded, XML) are now stored as plain strings instead of being base64-encoded, which makes them readable on the subscriber side. Binary payloads still get base64. Events also carry the HTTP method now, and verification failures log the request headers to help debug signature mismatches. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>