feat: SQLite DB with migrations replaces all JSON file storage

- Add db.php with PDO singleton, migration runner, and all helper functions
- Add migrations/001_initial_schema.sql (full schema)
- Add migrations/002_import_json.php (one-time JSON → DB importer)
- Add _incl/switch_tenant.php POST endpoint for tenant/centro switching
- Update tools.auth.php: DB-backed login, cookie auth, session reload, init_active_centro()
- Update all sysadmin pages (users, centros, aularios, invitations, reset_password) to use DB
- Update aulatek/index.php, aulario.php, supercafe.php, supercafe_edit.php to use DB
- Update aulatek/comedor.php and api/comedor.php to use DB
- Update aulatek/paneldiario.php: aulario config + comedor data from DB
- Update aulatek/proyectos.php: aulario config + sharing metadata from DB
- Update club/cal.php, index.php, edit_data.php, upload/upload.php to use DB
- Update account/index.php: rich profile, tenant list, aula list, session info, permissions
- Update pre-body.php account dropdown: shows active org + inline tenant switcher
- Update DATA_STRUCTURE.md to document DB approach and migration system

Co-authored-by: naielv <109038805+naielv@users.noreply.github.com>

Dockerfile.dev

@@ -8,7 +8,7 @@ FROM dunglas/frankenphp
 #     && rm -rf /var/lib/apt/lists/*
 
 # Configure PHP extensions
-RUN install-php-extensions gd opcache
+RUN install-php-extensions gd opcache pdo pdo_sqlite
 
 # Set working directory
 WORKDIR /var/www/html