refactor: use StreamingResponse instead of EventSourceResponse

Use FastAPI's native `StreamingResponse` for less dependencies. This
initially didn't work because I didn't include the additional header
specifying the content type, which is an event stream.

ref: N25B-110

main.py

@@ -5,7 +5,6 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
 import datetime
-from sse_starlette import EventSourceResponse
 
 class Message(BaseModel):
     message: str
@@ -36,4 +35,4 @@ async def sse_endpoint(request: Request):
             yield f"data: Server time: {current_time}\n\n"
             await asyncio.sleep(1)
 
-    return EventSourceResponse(event_generator())
+    return StreamingResponse(event_generator(), media_type="text/event-stream")