perf: allow disabling websocket per-message-deflate (#28613)

With delta streaming most websocket frames are tiny per-token deltas, and
per-message-deflate pays zlib work on every outgoing frame per subscriber for
near-zero gain there; under heavy streaming that shows up as measurable server
CPU. The frames that still benefit are the rare large ones (final message,
sources), and even a 100k token message is only a few hundred KB uncompressed,
which any network delivers without noticeable delay.

UVICORN_WS_PER_MESSAGE_DEFLATE=false (default true, current behavior) disables
the extension in every entry point: open-webui serve and dev, start.sh both
invocations, start_windows.bat and dev.sh. Verified against a running
instance: with the flag off the server declines the client-offered
permessage-deflate extension, with defaults it still negotiates it.

backend/dev.sh

@@ -1,3 +1,3 @@
 export CORS_ALLOW_ORIGIN="http://localhost:5173;http://localhost:8080"
 PORT="${PORT:-8080}"
-uvicorn open_webui.main:app --port $PORT --host 0.0.0.0 --forwarded-allow-ips "${FORWARDED_ALLOW_IPS:-*}" --reload
+uvicorn open_webui.main:app --port $PORT --host 0.0.0.0 --forwarded-allow-ips "${FORWARDED_ALLOW_IPS:-*}" --ws-per-message-deflate "${UVICORN_WS_PER_MESSAGE_DEFLATE:-true}" --reload