refactor: centralize configuration in backend/config/ and frontend/config/

- Move ldap_config.json from ./data/ to backend/config/
- Update users.py to read LDAP config from backend/config/ldap_config.json
- Create frontend/config/ directory for future frontend configs
- DATA_DIR still used for database and runtime files in ./data/
- Update .gitignore to track backend/config/ but ignore runtime data dirs

.gitignore

@@ -1,5 +1,7 @@
 backend/venv/
 backend/data/
+backend/logs/
+frontend/logs/
 __pycache__/
 *.pyc
 .env
@@ -7,4 +9,3 @@ __pycache__/
 aInventory-PROD*
 aInventory-PROD*.zip
 logs/
-backend/logs/