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.TemplateEngine.Tests/Flattening/FlatteningServiceTests.cs

@@ -200,7 +200,7 @@ public class FlatteningServiceTests
 
         var connections = new Dictionary<int, DataConnection>
         {
-            [100] = new("OPC-Server1", "OpcUa") { Id = 100, Configuration = "opc.tcp://localhost:4840" }
+            [100] = new("OPC-Server1", "OpcUa", 1) { Id = 100, Configuration = "opc.tcp://localhost:4840" }
         };
 
         var result = _sut.Flatten(