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>

web/js/i18n/en.js

@@ -201,6 +201,7 @@ export default {
   "pg.temp": "Temperature",
   "pg.maxTokens": "Max tokens",
   "pg.send": "Send",
+  "pg.stream": "Stream",
   "pg.sending": "Sending…",
   "pg.answer": "Answer",
   "pg.empty": "The answer and routing decision will appear here.",