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.

src/ScadaLink.Commons/Entities/Sites/DataConnection.cs

@@ -3,13 +3,15 @@ namespace ScadaLink.Commons.Entities.Sites;
 public class DataConnection
 {
     public int Id { get; set; }
+    public int SiteId { get; set; }
     public string Name { get; set; }
     public string Protocol { get; set; }
     public string? Configuration { get; set; }
 
-    public DataConnection(string name, string protocol)
+    public DataConnection(string name, string protocol, int siteId)
     {
         Name = name ?? throw new ArgumentNullException(nameof(name));
         Protocol = protocol ?? throw new ArgumentNullException(nameof(protocol));
+        SiteId = siteId;
     }
 }

src/ScadaLink.Commons/Entities/Sites/SiteDataConnectionAssignment.cs

@@ -1,8 +0,0 @@
-namespace ScadaLink.Commons.Entities.Sites;
-
-public class SiteDataConnectionAssignment
-{
-    public int Id { get; set; }
-    public int SiteId { get; set; }
-    public int DataConnectionId { get; set; }
-}