Add comprehensive demo data loading logic and .env.demo configuration

- Implemented `load_demo_data` to populate organizations, users, and relationships from `demo_data.json`.
- Refactored database initialization to handle demo-specific passwords and multi-entity creation in demo mode.
- Added `demo_data.json` with sample organizations and users for better demo showcase.
- Introduced `.env.demo` to simplify environment setup for demo scenarios.
- Updated `.gitignore` to include `.env.demo` while keeping other `.env` files excluded.

.gitignore

@@ -268,6 +268,7 @@ celerybeat.pid
 .env
 .env.*
 !.env.template
+!.env.demo
 .venv
 env/
 venv/