feat(web): stream chat replies token-by-token (M3)

- llm.chat_call_stream: streaming generator for all 3 backends (Ollama NDJSON,
  OpenAI/MI50 SSE), accumulating tool-call fragments by index.
- chat.respond_stream: mirrors respond()'s tool loop and persistence/compaction,
  yielding ("delta", text) / ("tool", name) / ("done", reply).
- POST /v1/chat/stream: SSE endpoint; blocking generator bridged to async via a
  worker thread + asyncio.Queue. Old completions endpoint kept as fallback.
- Client streams into a live bubble with a blinking caret; rAF-throttled render
  (no full re-parse per token) and instant scroll during stream — fixes iOS
  Safari ghosting from per-token smooth-scroll. Falls back to the blocking
  endpoint only if nothing streamed (no double-persist).

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

lyra/web/server.py

@@ -111,6 +111,45 @@ def create_app() -> FastAPI:
             ],
         }
 
+    @app.post("/v1/chat/stream")
+    async def chat_stream(request: Request) -> StreamingResponse:
+        """Server-Sent Events: stream Lyra's reply token-by-token.
+
+        `chat.respond_stream` is a blocking generator (httpx/openai), so it runs in
+        a worker thread and bridges chunks to this async generator via a queue.
+        """
+        body = await request.json()
+        session_id = body.get("sessionId") or "default"
+        backend = _backend_for(body.get("backend"))
+        user_msg = _last_user_message(body.get("messages", []))
+        model_override = body.get("model") or None
+        memory.ensure_session(session_id)
+
+        async def gen():
+            loop = asyncio.get_running_loop()
+            q: asyncio.Queue = asyncio.Queue()
+            done = object()
+
+            def produce():
+                try:
+                    for event in chat.respond_stream(session_id, user_msg, backend, model_override):
+                        loop.call_soon_threadsafe(q.put_nowait, event)
+                except Exception as exc:  # surface to the client stream, don't hang
+                    logbus.log("error", "chat stream failed", session=session_id, error=str(exc))
+                    loop.call_soon_threadsafe(q.put_nowait, ("error", str(exc)))
+                finally:
+                    loop.call_soon_threadsafe(q.put_nowait, done)
+
+            loop.run_in_executor(None, produce)
+            while True:
+                item = await q.get()
+                if item is done:
+                    break
+                ev, payload = item
+                yield f"data: {json.dumps({'type': ev, 'payload': payload})}\n\n"
+
+        return StreamingResponse(gen(), media_type="text/event-stream")
+
     @app.get("/logs")
     async def logs_page() -> FileResponse:
         """Full-page, mobile-friendly live log viewer (separate from the chat UI)."""