Add nginx X-Accel-Redirect support, Redis cache provider, and author access fix

[Erik-NA]

Summary

• nginx X-Accel-Redirect: FileHandler sends X-Accel-Redirect instead of X-Sendfile on nginx, with a /uam-files/ internal location prefix to avoid redirect loops. SettingsController::isXSendFileAvailable() returns true on nginx and shows the correct server-specific label in the UI (X-Accel-Redirect vs X-Sendfile). Apache gets a runtime fallback via apache_get_modules() if mod_xsendfile is absent.
• nginx location regex fix: FileProtection::getDirectoryMatch() uses \d{4} instead of [0-9]{4} — nginx cannot parse character classes with quantifiers in location directives.
• Redis cache provider: New RedisCacheProvider using the WordPress object cache API (wp_cache_*). Registered automatically when wp_using_ext_object_cache() is true. No configuration needed beyond a standard Redis Object Cache plugin.
• Author access fix: AccessHandler::hasAuthorAccess() now guards against $currentUserId === 0, preventing unauthenticated users from gaining access to posts imported without an author (post_author = 0).

nginx configuration required

location ~ "^/wp-content/uploads/\d{4}/\d{2}" {
    rewrite ^([^?]*)$ /index.php?uamfiletype=attachment&uamgetfile=$1 last;
    rewrite ^(.*)\?(((?!uamfiletype).)*)$ /index.php?uamfiletype=attachment&uamgetfile=$1&$2 last;
    rewrite ^(.*)\?(.*)$ /index.php?uamgetfile=$1&$2 last;
}
location /uam-files/ {
    internal;
    alias /var/www/html/;
}

Replaces #432 (splitting as requested).