Add design doc: deploy artifacts to sites, remove config DB dependency

docs/plans/2026-03-17-deploy-artifacts-remove-configdb-design.md

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+# Design: Deploy Artifacts to Sites — Remove Config DB Dependency
+
+**Date:** 2026-03-17
+**Status:** Approved
+
+## Problem
+
+`ExternalSystemClient`, `DatabaseGateway`, and `NotificationDeliveryService` query the central config DB at runtime from site servers. The data is already deployed to local SQLite via `DeployArtifactsCommand` but never read back. Sites should be fully self-contained after artifact deployment.
+
+## Changes
+
+### 1. Site-local repository implementations
+
+Create `SiteExternalSystemRepository` and `SiteNotificationRepository` that read from `SiteStorageService` (SQLite) instead of EF Core config DB repositories. Implement `IExternalSystemRepository` and `INotificationRepository` interfaces backed by SQLite queries.
+
+### 2. Remove AddConfigurationDatabase from Site role
+
+Replace `AddConfigurationDatabase` in Program.cs Site role with site-local repository registrations. Site no longer needs a SQL Server connection string.
+
+### 3. Add "Deploy Artifacts" button to Admin UI
+
+On the Sites page, add a "Deploy Artifacts" button per site that:
+- Collects all external systems, database connections, notification lists + recipients, SMTP configs from config DB
+- Sends `DeployArtifactsCommand` to the site via `CommunicationService`
+- Shows success/failure toast
+
+### 4. Include data connections in artifact deployment
+
+Extend `DeployArtifactsCommand` to include data connection definitions (OPC UA endpoints, etc). Add `StoreDataConnectionDefinitionAsync` to `SiteStorageService`.
+
+### 5. No change to FlattenedConfiguration
+
+The DCL already reads connection config from `FlattenedConfiguration.Connections` dict, populated during flattening from config DB. This stays as-is.
+
+## Outcome
+
+After artifact deployment, sites operate without any config DB access. The config DB connection string is removed from `appsettings.Site.json`.