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/env.py

@@ -150,6 +150,11 @@ INSTANCE_ID = os.getenv('INSTANCE_ID', str(uuid4()))
 
 ENABLE_DB_MIGRATIONS = os.getenv('ENABLE_DB_MIGRATIONS', 'True').lower() == 'true'
 
+# Swap the JSON encoder/decoder used across the app (HTTP request bodies, JSONResponse
+# bodies, upstream provider responses, socket.io payloads) from the stdlib `json` module
+# to orjson. Faster, but stricter: see open_webui/utils/json_codec.py for the differences.
+ENABLE_ORJSON = os.getenv('ENABLE_ORJSON', 'False').lower() == 'true'
+
 
 # Function to parse each section
 def parse_section(section):