perfetto.dev serves all assets uncompressed

The App Engine proxy copies the stored object's `content_encoding` onto every response. Since `download_as_bytes()` always returns decompressed bytes, this stamps a bogus `Content-Encoding` (either `gzip` or the literal `None`), which makes the App Engine frontend skip its own gzip.

Result: HTML, JS, CSS, and the search index all ship uncompressed (~2.5-3x larger than they should be).

For the docs search we worked around this in https://github.com/google/perfetto/pull/6702 by shipping the index pre-gzipped and decompressing it in the browser. That workaround should be removed once the proxy is fixed.

Better fix: align the docs-site serving with how the UI server already handles compression.

Inspect the App Engine proxy and compare its response compression handling with the UI server. Review PR 6702 to identify the pre-gzipped search-index workaround. Done means HTML, JavaScript, CSS, and search-index assets are served compressed without bogus Content-Encoding headers, allowing that workaround to be removed.

Written by the indexing model from the issue text.