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/utils/json_codec.py

@@ -0,0 +1,50 @@
+"""The app-wide JSON codec, selected by the ``ENABLE_ORJSON`` env var.
+
+Every module that would otherwise reach for stdlib ``json`` imports ``JSONCodec``
+from here, so the whole app switches implementation from a single flag. With the
+flag off these are stdlib ``json`` and engineio's codec verbatim, so the default
+behaviour is exactly what it was before orjson entered the picture.
+"""
+
+from __future__ import annotations
+
+import json as stdlib_json
+
+from engineio import json as engineio_json
+
+from open_webui.env import ENABLE_ORJSON
+
+if ENABLE_ORJSON:
+    import orjson
+
+    class ORJSONCodec:
+        """stdlib-``json``-compatible codec backed by orjson.
+
+        Anything orjson rejects (non-str dict keys, ints beyond 64 bits, ``NaN``
+        literals) falls back to engineio's stdlib-based codec, which keeps its
+        oversized-integer guard for untrusted client payloads.
+        """
+
+        JSONDecodeError = engineio_json.JSONDecodeError
+
+        @staticmethod
+        def dumps(obj, *args, **kwargs):
+            try:
+                return orjson.dumps(obj).decode('utf-8')
+            except (TypeError, ValueError):
+                return engineio_json.dumps(obj, *args, **kwargs)
+
+        @staticmethod
+        def loads(s, *args, **kwargs):
+            try:
+                return orjson.loads(s)
+            except (TypeError, ValueError):
+                return engineio_json.loads(s, *args, **kwargs)
+
+    # Drop-in for stdlib ``json``: ``JSONCodec.dumps`` / ``JSONCodec.loads``.
+    JSONCodec = ORJSONCodec
+    # Codec handed to the socket.io/engineio managers, which default to their own.
+    SOCKETIO_JSON = ORJSONCodec
+else:
+    JSONCodec = stdlib_json
+    SOCKETIO_JSON = engineio_json