chore: Replace specific domain with generic example URL

Changes:
- docker-compose.yml: https://nlcc.rydertech.us → https://church.example.com
- Dockerfile: https://nlcc.rydertech.us → https://church.example.com
- README.md: https://nlcc.rydertech.us → https://church.example.com

Rationale:
- Makes the repository more generic and reusable
- Users won't be confused by site-specific URLs
- Follows standard documentation practices using example.com
- Clearer that SITE_URL needs to be customized for each deployment

The example.com domain is reserved by IANA for documentation purposes,
making it the perfect choice for configuration templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -33,7 +33,7 @@ This application uses environment variables configured directly in `docker-compo
 
 | Variable | Description | Default | Required |
 |----------|-------------|---------|----------|
-| `SITE_URL` | Public URL where the app is hosted (used for QR codes) | `https://nlcc.rydertech.us` | Yes |
+| `SITE_URL` | Public URL where the app is hosted (used for QR codes) | `https://church.example.com` | Yes |
 | `AUTH_SECRET` | Secret key for authentication sessions | **Auto-generated** | No |
 | `ADMIN_USERNAME` | Initial admin login username | `admin` | No |
 | `ADMIN_PASSWORD` | Initial admin login password | **Auto-generated** | No |