perf: optional orjson JSON codec behind ENABLE_ORJSON (#27583)

Swap the JSON encoder/decoder used across the backend from stdlib json to
orjson when ENABLE_ORJSON is set — HTTP request bodies, JSONResponse
bodies, upstream provider responses, SSE chunks, and socket.io/Redis
payloads.

The flag defaults to off, in which case the app uses stdlib json and
engineio's codec verbatim, so default behaviour is unchanged.

- json_codec exports JSONCodec (stdlib json or the orjson codec) and
  SOCKETIO_JSON (engineio's codec or the orjson codec); call sites import
  JSONCodec and stay implementation-agnostic
- apply_orjson_http_json() is a no-op when the flag is off, leaving
  starlette's Request.json / JSONResponse.render untouched
- the orjson codec falls back to the stdlib for inputs orjson rejects
  (non-str dict keys, ints beyond 64 bits, NaN literals)
- orjson is imported only when the flag is on
- FastAPI(default_response_class=...) is deliberately not used: an
  explicit default disables the Pydantic direct-to-bytes fast path for
  response_model routes

backend/open_webui/main.py

@@ -229,6 +229,7 @@ from open_webui.utils.chat_variables import (
     normalize_chat_variables,
 )
 from open_webui.utils.embeddings import generate_embeddings
+from open_webui.utils.json_response import apply_orjson_http_json
 from open_webui.utils.logger import start_logger
 from open_webui.utils.middleware import (
     background_tasks_handler,
@@ -456,6 +457,10 @@ async def lifespan(app: FastAPI):
     await publish_event(app, EVENTS.SYSTEM_SHUTDOWN_COMPLETED, source='system')
 
 
+# Opt-in (ENABLE_ORJSON): orjson for request-body parsing and JSONResponse bodies;
+# response_model routes keep FastAPI's Pydantic fast path either way.
+apply_orjson_http_json()
+
 app = FastAPI(
     title='Open WebUI',
     docs_url='/docs' if ENV == 'dev' else None,