feat: increase temporary document size limit to 1MB

[OhMyMndy]

I had to increase the limit because some pages were really large.

[supahgreg]

Thanks!

I'm thinking this might be a good opportunity to introduce a config option-- keeping the default at 512000 but allowing an override (e.g. environment variable TTRSS_AF_READABILITY_MAX_DOC_SIZE=1024000).

AUTH_OIDC_VALIDATE_INTERVAL in https://github.com/tt-rss/tt-rss-plugin-auth-oidc/blob/main/init.php could be used as an example, if you're interested in making the change.