Papra Was Running. SQLite Still Couldn't Write.
Papra looked healthy on my NAS while registration failed. The browser hid the useful detail: SQLite belonged to the wrong user.
Web Server
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Running PHP 5.6 with Apache on El Capitan
The Apache module path was the easy-to-miss part of running Homebrew's PHP 5.6 on macOS El Capitan. Here is the full setup.
Upgrading WordPress to PHP 7: Configuration Changes
Ubuntu 16.04 ships with PHP 7, which broke my WordPress. Here's how to upgrade PHP config and point nginx to the new php-fpm socket.
Keep exploring