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/open_webui/env.py

@@ -443,6 +443,9 @@ try:
 except (ValueError, TypeError):
     UVICORN_WORKERS = 1
 
+# tiny delta-stream frames make per-frame websocket compression CPU-bound, allow opting out (true/false)
+UVICORN_WS_PER_MESSAGE_DEFLATE = os.getenv('UVICORN_WS_PER_MESSAGE_DEFLATE', 'True').lower() == 'true'
+
 ####################################
 # WEBSOCKET SUPPORT
 ####################################