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/es.js

@@ -200,6 +200,7 @@ export default {
   "pg.temp": "Temperatura",
   "pg.maxTokens": "Tokens máx.",
   "pg.send": "Enviar",
+  "pg.stream": "Streaming",
   "pg.sending": "Enviando…",
   "pg.answer": "Respuesta",
   "pg.empty": "La respuesta y la decisión de enrutamiento aparecerán aquí.",