refactor: centralize ndjson streaming transport

app/backend/http/handler.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import json
 import mimetypes
 import secrets
+from collections.abc import Iterable
 from http import HTTPStatus
 from http.cookies import SimpleCookie
 from typing import Any
@@ -142,5 +143,33 @@ class HttpTransportMixin:
         self.end_headers()
         self.wfile.write(content)
 
+    def _write_stream_event(self, payload: dict[str, Any]) -> None:
+        self.wfile.write(
+            (json.dumps(payload, ensure_ascii=False, separators=(",", ":")) + "\n").encode("utf-8")
+        )
+        self.wfile.flush()
+
+    def send_ndjson_stream(
+        self,
+        events: Iterable[dict[str, Any]],
+        error_types: tuple[type[Exception], ...],
+    ) -> None:
+        self.send_response(HTTPStatus.OK)
+        self.send_header("Content-Type", "application/x-ndjson; charset=utf-8")
+        self.send_header("Cache-Control", "no-cache, no-transform")
+        self.send_header("X-Accel-Buffering", "no")
+        self.send_header("Connection", "close")
+        self.end_headers()
+        try:
+            for event in events:
+                self._write_stream_event(event)
+            self._write_stream_event({"type": "done"})
+        except error_types as exc:
+            self._write_stream_event({"type": "error", "error": str(exc)})
+        except (BrokenPipeError, ConnectionResetError):
+            pass
+        finally:
+            self.close_connection = True
+
     def log_message(self, format_string: str, *args: Any) -> None:
         print(f"[{self.log_date_time_string()}] {format_string % args}")