Add SQLite groundwork: Lib\Db, migrations, and a project-owned data dir

Lib\Db (novaconium/lib/Db.php) is a thin, no-ORM PDO wrapper — lazy-connect
like Lib\Csrf, Db::query() as the only query-running helper (prepared
statements only, no interpolation shortcut, per Lib\Input's existing
security stance). Plain .sql migrations under App/migrations/, applied in
filename order and tracked in an auto-created schema_migrations table, run
automatically on first connection or via novaconium/bin/migrate.php.

Data lives in a new top-level data/ directory rather than novaconium/ or
public/ — outside public/ so it's never web-accessible, and outside
novaconium/ since that directory gets wholesale-replaced by the "Updating
the framework" workflow, which would otherwise destroy it on every update.

New config keys: db_driver (only 'sqlite' implemented), db_path,
db_migrations_dir. Documented at /admin/docs/database and in AGENTS.md.
Closes the "SQLite groundwork" backlog item in novaconium/ISSUES.md.

.gitignore

@@ -1,4 +1,8 @@
 /public/cache/*
 !/public/cache/.gitkeep
 /novaconium/contact-log.txt
+/data/*.sqlite
+/data/*.sqlite-journal
+/data/*.sqlite-wal
+/data/*.sqlite-shm
 .claude/