Refactor admin panel and add migration documentation

- Add comprehensive migration guides (DEPLOYMENT_MIGRATION.md, DIRECTORY_STRUCTURE.md, MIGRATION_CHECKLIST.md)
- Refactor admin panel: split add.php, create reusable header/footer
- Update styles: admin.css, common.css, main.css
- Improve public pages: index.php, memoire.php
- Reorganize database documentation into database/docs/
- Update .gitignore and justfile

This prepares for migration to public/ directory structure

.gitignore

@@ -1,27 +1,13 @@
+# Vendor directory (third-party code)
 vendor/
 compose.lock
 
 ### Test databases ###
-formulaire/test.db
-db/test.db
-
-### Data et Mémoire###
-formulaire/data/yaml/*
-formulaire/data/content/*
-formulaire/data/cover/*
-formulaire/data/theses/*
-formulaire/data/covers/*
-
-front-backend/data/yaml/*
-front-backend/data/content/*
-front-backend/data/cover/*
+database/test.db
 
 ### Logs ###
 formulaire/error.log
-
-
-# Vendor directory (third-party code)
-vendor/
+lib/cache/rate_limit/
 
 # OS files
 .DS_Store