feat: streaming (SSE) end-to-end

- Provider::chat_stream; OpenAI provider streams via stream_options.include_usage
- pipeline.run_stream: shared route/select, failover before first byte
- /v1/chat/completions forwards SSE verbatim with X-Cortiq-* headers; usage tapped for stats
- admin /test/stream + live streaming in Playground (stream toggle, 7 langs)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -48,6 +48,7 @@ getrandom = "0.2"
 # streaming / async utils
 futures = "0.3"
 async-trait = "0.1"
+async-stream = "0.3"
 bytes = "1"
 tokio-stream = "0.1"