feat: implement working Elasticsearch gzip compression

[strawgate]

Problem

Current compress: true config option does nothing - no Content-Encoding header is set and body is not actually compressed. Commit 3c60f84 disabled it after bug #2625.

Why

Users enabling compression expect data to be compressed, but ES receives uncompressed body with no header.

Requirements

1. Add gzip compression in send.rs (use existing GzEncoder code)
2. Add Content-Encoding: gzip header when config.compress == true
3. Preserve buffer reuse optimization from original design
4. Test against real Elasticsearch

Files

• crates/ffwd-output/src/elasticsearch/send.rs - add header
• crates/ffwd-output/src/elasticsearch/transport.rs - already passes body