Caddy: route SSE and binary requests to backend, not just JSON

The previous matcher required Accept to contain application/json,
which made browser-driven streaming chat requests (Accept:
text/event-stream) fall through to the Next.js frontend and return
the 404 HTML page as the chat reply.

Invert the matcher: route to backend when the path matches AND the
Accept header does NOT contain text/html. Browser navigation always
sends text/html; JSON fetches, SSE streams, and binary downloads
never do. One rule covers all three.

Operators apply this on a running deployment with:

  sudo bash bin/pip-write-caddyfile.sh