perf: 40% LESS CPU usage: cut per-instance CPU cost of shared socket.io Redis pub/sub channel (#27282)

* perf: cut per-instance CPU cost of shared socket.io Redis pub/sub channel

Profiling a multi-instance deployment (py-spy --gil) showed ~44% of worker
CPU in the socket.io pub/sub listener. Two causes, two fixes:

- Add hiredis so redis-py parses the RESP protocol in C instead of pure
  Python (redis/_parsers/resp3.py alone accounted for ~28% of GIL samples;
  redis-py auto-selects the hiredis parser when importable).

- Subclass AsyncRedisManager to drop emits whose target room has no local
  participants before upstream _handle_emit re-encodes the full packet.
  Every instance receives every emit published on the shared channel, so
  with N instances all but the hosting one were paying full packet
  re-serialization per message just to deliver it to nobody. Broadcasts
  (room=None) are unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q9CQ9qnp3sZGYQQwztsCJT

* fix: restrict pub/sub emit early-out to string rooms

Adversarial review against python-socketio 5.16.2 found one divergence
from upstream: for a degenerate empty-sequence room (emit to room=[]) on
an instance whose namespace has no local clients, the filter's
get_participants probe raises IndexError from room[0] where upstream
returns silently at the namespace guard and still publishes to Redis.
Open WebUI only ever emits to scalar string rooms or room=None, so the
case is unreachable today; guard on isinstance(room, str) anyway so any
non-string room shape passes through to upstream behavior unchanged.

Every open-webui emit uses a string room, so the fast path still covers
all real traffic.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q9CQ9qnp3sZGYQQwztsCJT

* Update requirements.txt

* Update pyproject.toml

* Update requirements-min.txt

---------

Co-authored-by: Claude <noreply@anthropic.com>

backend/requirements.txt

@@ -30,7 +30,8 @@ psycopg[binary]==3.3.4
 alembic==1.18.4
 
 pycrdt==0.13.1
-redis==8.0.0
+redis==8.0.1
+hiredis==3.4.0
 
 APScheduler==3.11.2
 RestrictedPython==8.2