fix: convert SecurityHeadersMiddleware to pure ASGI (#26924)

SecurityHeadersMiddleware was the last middleware in the stack still
subclassing BaseHTTPMiddleware, after CommitSession, AuthToken,
WebsocketUpgradeGuard and Redirect were all moved to pure ASGI in
utils/asgi_middleware.py. BaseHTTPMiddleware re-buffers the response
body through an anyio task group, which has known issues with
streaming and Content-Length-bearing responses (e.g. the FileResponse
returned by /api/v1/audio/speech).

Reimplement it as a pure-ASGI middleware that stamps the configured
security headers onto the http.response.start message via
MutableHeaders and forwards all body chunks untouched, matching the
pattern already used by its four siblings. set_security_headers() and
all its helpers are unchanged.

Co-authored-by: classic298 <classic298@users.noreply.github.com>

backend/open_webui/utils/security_headers.py

@@ -2,15 +2,33 @@ import os
 import re
 from typing import Dict
 
-from fastapi import Request
-from starlette.middleware.base import BaseHTTPMiddleware
+from starlette.datastructures import MutableHeaders
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
 
 
-class SecurityHeadersMiddleware(BaseHTTPMiddleware):
-    async def dispatch(self, request: Request, call_next):
-        response = await call_next(request)
-        response.headers.update(set_security_headers())
-        return response
+class SecurityHeadersMiddleware:
+    """Apply configured security headers to every HTTP response.
+
+    Pure ASGI to avoid BaseHTTPMiddleware's response re-buffering. See
+    open_webui.utils.asgi_middleware for the rationale.
+    """
+
+    def __init__(self, app: ASGIApp) -> None:
+        self.app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope['type'] != 'http':
+            await self.app(scope, receive, send)
+            return
+
+        async def send_with_security_headers(message: Message) -> None:
+            if message['type'] == 'http.response.start':
+                headers = MutableHeaders(scope=message)
+                for key, value in set_security_headers().items():
+                    headers[key] = value
+            await send(message)
+
+        await self.app(scope, receive, send_with_security_headers)
 
 
 def set_security_headers() -> Dict[str, str]: