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.

.env.demo

@@ -0,0 +1,31 @@
+# Common settings
+PROJECT_NAME=App
+VERSION=1.0.0
+
+# Database settings
+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=postgres
+POSTGRES_DB=app
+POSTGRES_HOST=db
+POSTGRES_PORT=5432
+DATABASE_URL=postgresql://postgres:postgres@db:5432/app
+
+# Backend settings
+BACKEND_PORT=8000
+# CRITICAL: Generate a secure SECRET_KEY for production!
+# Generate with: python -c 'import secrets; print(secrets.token_urlsafe(32))'
+# Must be at least 32 characters
+SECRET_KEY=demo_secret_key_for_testing_only_do_not_use_in_prod
+ENVIRONMENT=development
+DEMO_MODE=true
+DEBUG=true
+BACKEND_CORS_ORIGINS=["http://localhost:3000"]
+FIRST_SUPERUSER_EMAIL=admin@example.com
+# IMPORTANT: Use a strong password (min 12 chars, mixed case, digits)
+# Default weak passwords like 'Admin123' are rejected
+FIRST_SUPERUSER_PASSWORD=Admin123!
+
+# Frontend settings
+FRONTEND_PORT=3000
+NEXT_PUBLIC_API_URL=http://localhost:8000
+NODE_ENV=development