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>

src/providers/mod.rs

@@ -7,8 +7,16 @@
 pub mod anthropic;
 pub mod openai;
 
+use crate::error::GatewayError;
 use crate::model::{ChatRequest, ChatResponse};
 use async_trait::async_trait;
+use bytes::Bytes;
+use futures::stream::BoxStream;
+
+/// A streaming chat response: raw Server-Sent-Events byte chunks in OpenAI wire
+/// format (`data: {json}\n\n` … `data: [DONE]\n\n`). The gateway forwards these
+/// to the client verbatim while tapping the final `usage` for statistics.
+pub type ChatStream = BoxStream<'static, crate::error::Result<Bytes>>;
 
 /// Model capabilities — checked at selection time (whether tools/vision are needed).
 #[derive(Clone, Debug, Default)]
@@ -41,6 +49,13 @@ pub trait Provider: Send + Sync {
     /// Non-streaming chat call.
     async fn chat(&self, req: ChatRequest) -> crate::error::Result<ChatResponse>;
 
-    // TODO(v0.1): async fn chat_stream(...) -> Result<ChatStream> (SSE).
-    // TODO(v0.1): async fn embed(...) -> Result<EmbedResponse>.
+    /// Streaming chat call (SSE). Default: not supported (override per provider).
+    async fn chat_stream(&self, _req: ChatRequest) -> crate::error::Result<ChatStream> {
+        Err(GatewayError::UpstreamUnavailable(format!(
+            "streaming is not supported by provider '{}'",
+            self.id()
+        )))
+    }
+
+    // TODO(v0.2): async fn embed(...) -> Result<EmbedResponse>.
 }

src/providers/openai.rs

@@ -154,4 +154,35 @@ impl Provider for OpenAiProvider {
             },
         })
     }
+
+    async fn chat_stream(&self, req: ChatRequest) -> Result<super::ChatStream> {
+        use futures::StreamExt;
+        let mut body = self.to_wire(&req);
+        body["stream"] = serde_json::Value::Bool(true);
+        // ask OpenAI-compatible servers to include token usage in the final chunk
+        body["stream_options"] = serde_json::json!({ "include_usage": true });
+        let mut r = self
+            .http
+            .post(format!("{}/chat/completions", self.base_url))
+            .json(&body);
+        if let Some(key) = &self.api_key {
+            r = r.bearer_auth(key);
+        }
+        let resp = r
+            .send()
+            .await
+            .map_err(|e| GatewayError::UpstreamUnavailable(e.to_string()))?;
+        if !resp.status().is_success() {
+            let status = resp.status();
+            let err_body = resp.text().await.unwrap_or_default();
+            return Err(GatewayError::UpstreamUnavailable(format!(
+                "provider {} returned HTTP {}: {}",
+                self.id, status, err_body
+            )));
+        }
+        let stream = resp
+            .bytes_stream()
+            .map(|res| res.map_err(|e| GatewayError::UpstreamUnavailable(e.to_string())));
+        Ok(Box::pin(stream))
+    }
 }