chore(psr4): phase 3 (Core hubs) — namespace Core/Database

Move Core/Database into XcVm\Core\Database (DatabaseHandler, Database,
MigrationRunner, QueryHelper). First of the Core hub sub-layers.

- Namespace the 4 classes; DatabaseHandler extends Database (same namespace);
  built-ins/ioncube qualified (\PDO, \PDOException, \Exception, \Throwable,
  \XC_VM); Database keeps its 'use XcVm\Core\Logging\FileLogger;'.
- Add 'use XcVm\Core\Database\...;' to referencing files (DatabaseHandler 51,
  Database 64, QueryHelper 29, MigrationRunner 3) + 2 test files (PHPStan does not
  analyse tests/, so PHPUnit is the gate there).
- Rewrite pre-existing leading-backslash global refs (\DatabaseHandler etc., e.g.
  in @param docblocks of ResellerApiDispatcher/ResellerTableRenderer) to the full
  FQCN \XcVm\Core\Database\... — a 'use' import does not cover a leading-\
  reference. Done with a lookbehind so FQCN continuations and use-lines are intact.
- phpstan-baseline.neon regenerated (292→292; pre-existing Database/migration_logic
  findings re-anchored after class names in messages gained the namespace).

Verified: php -l clean; PHPStan no errors; PHPUnit 295/295.

tests/Unit/QueryHelperTest.php

@@ -1,5 +1,7 @@
 <?php
 
+use XcVm\Core\Database\Database;
+use XcVm\Core\Database\QueryHelper;
 use PHPUnit\Framework\TestCase;
 
 /**