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.

novaconium/bin/migrate.php

@@ -0,0 +1,13 @@
+<?php
+
+use Lib\Db;
+
+require __DIR__ . '/../autoload.php';
+
+// Db::connection() applies any pending App/migrations/*.sql as a side
+// effect of opening the connection (see Lib\Db::migrate()) — this script
+// just triggers that explicitly, e.g. from a deploy script, without
+// serving a request first.
+Db::connection();
+
+echo "Migrations applied.\n";