Stop forcing Access-Control-Allow-Origin to $scheme://$host. This default broke Element (element.infinito.nexus) -> Synapse (matrix.infinito.nexus) CORS and blocked login. Now ACAO is only set when 'aca_origin' is provided; otherwise we defer to the upstream app (e.g., Synapse) to emit correct CORS headers. Also convert top comments to Jinja block comment.
Discussion & debugging details: https://chatgpt.com/share/68de2236-4aec-800f-adc5-d025922c8753