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>
