Replace SiteDataConnectionAssignment join table with a direct SiteId FK on DataConnection,
simplifying the data model, repositories, UI, CLI, and deployment service.
- SiteExternalSystemRepository and SiteNotificationRepository registered in Site DI
- Removed AddConfigurationDatabase from Site role in Program.cs
- Removed ConfigurationDb from appsettings.Site.json
- ArtifactDeploymentService collects all 6 artifact types including data connections and SMTP