refactor: simplify data connections from many-to-many site assignment to direct site ownership

Replace SiteDataConnectionAssignment join table with a direct SiteId FK on DataConnection,
simplifying the data model, repositories, UI, CLI, and deployment service.

tests/ScadaLink.DeploymentManager.Tests/ArtifactDeploymentServiceTests.cs

@@ -37,9 +37,8 @@ public class ArtifactDeploymentServiceTests
         _siteRepo.GetAllSitesAsync().Returns(new List<Site>());
 
         var service = CreateService();
-        var command = CreateCommand();
 
-        var result = await service.DeployToAllSitesAsync(command, "admin");
+        var result = await service.DeployToAllSitesAsync("admin");
 
         Assert.True(result.IsFailure);
         Assert.Contains("No sites", result.Error);