Deployment configuration

Purpose: Give one deployment a single, persistent answer to "which site is this and where is its data" for the web app, CLI, cron and cache workers. Scenario: QDB puts FORUM_SITE_ID => 'qdb' and its paths in forum-private/qdb/secrets.php beside its qdb checkout. Apache, ./v3 commands and the cron worker all serve QDB with no SetEnv, and a git reset --hard changes nothing. An environment variable still overrides the file; ./v3 status shows each value's source and warns when tracked public/.htaccess carries local edits. Sources: lookup (source unavailable), private file location (source unavailable), runbook.