[IMP] reorganize project directory structure

Restructure the project for better organization and maintainability:

New structure:
  ./upgrade.sh              - Main entry point (unchanged)
  ./lib/common.sh           - Shared bash functions
  ./lib/python/             - Python utility scripts
  ./scripts/                - Workflow scripts (prepare_db, finalize_db)
  ./config/                 - Configuration files (compose.yml)
  ./versions/{13..18}.0/    - Version-specific migration scripts

File renames:
  - pre_migration_view_checking.py -> lib/python/check_views.py
  - post_migration_fix_duplicated_views.py -> lib/python/fix_duplicated_views.py
  - post_migration_cleanup_obsolete_modules.py -> lib/python/cleanup_modules.py

Benefits:
  - Single entry point visible at root level
  - Clear separation between shared code, scripts, and config
  - Shorter, cleaner Python script names (context given by caller)
  - Easier navigation and maintenance

versions/13.0/pre_upgrade.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+set -euo pipefail
+
+echo "Prepare migration to 13.0..."
+
+# Copy database
+copy_database ou12 ou13 ou13 || exit 1
+
+# Execute SQL pre-migration commands
+PRE_MIGRATE_SQL=$(cat <<'EOF'
+/* Add analytic_policy column as openupgrade script is waiting for it whereas it doesn't existe since v12.  */
+ALTER TABLE public.account_account_type ADD analytic_policy varchar NULL;
+
+/* The model in missing on some website_sale data */
+UPDATE ir_model_data SET model = 'ir.ui.view' WHERE module = 'website_sale' AND name = 'recommended_products';
+UPDATE ir_model_data SET model = 'ir.ui.view' WHERE module = 'website_sale' AND name = 'product_comment';
+EOF
+)
+query_postgres_container "$PRE_MIGRATE_SQL" ou13 || exit 1
+
+# Copy filestores
+copy_filestore ou12 ou12 ou13 ou13 || exit 1
+
+echo "Ready for migration to 13.0!"