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; }
-}

src/ScadaLink.Commons/Interfaces/Repositories/ICentralUiRepository.cs

@@ -10,7 +10,7 @@ public interface ICentralUiRepository
 {
     Task<IReadOnlyList<Site>> GetAllSitesAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DataConnection>> GetDataConnectionsBySiteIdAsync(int siteId, CancellationToken cancellationToken = default);
-    Task<IReadOnlyList<SiteDataConnectionAssignment>> GetAllSiteDataConnectionAssignmentsAsync(CancellationToken cancellationToken = default);
+    Task<IReadOnlyList<DataConnection>> GetAllDataConnectionsAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Template>> GetTemplateTreeAsync(CancellationToken cancellationToken = default);
     Task<IReadOnlyList<Instance>> GetInstancesFilteredAsync(int? siteId = null, int? templateId = null, string? searchTerm = null, CancellationToken cancellationToken = default);
     Task<IReadOnlyList<DeploymentRecord>> GetRecentDeploymentsAsync(int count, CancellationToken cancellationToken = default);

src/ScadaLink.Commons/Interfaces/Repositories/ISiteRepository.cs

@@ -24,11 +24,6 @@ public interface ISiteRepository
     Task UpdateDataConnectionAsync(DataConnection connection, CancellationToken cancellationToken = default);
     Task DeleteDataConnectionAsync(int id, CancellationToken cancellationToken = default);
 
-    // Site-Connection Assignments
-    Task<SiteDataConnectionAssignment?> GetSiteDataConnectionAssignmentAsync(int siteId, int dataConnectionId, CancellationToken cancellationToken = default);
-    Task AddSiteDataConnectionAssignmentAsync(SiteDataConnectionAssignment assignment, CancellationToken cancellationToken = default);
-    Task DeleteSiteDataConnectionAssignmentAsync(int id, CancellationToken cancellationToken = default);
-
     // Instances (for deletion constraint checks)
     Task<IReadOnlyList<Instance>> GetInstancesBySiteIdAsync(int siteId, CancellationToken cancellationToken = default);
 

src/ScadaLink.Commons/Messages/Management/DataConnectionCommands.cs

@@ -1,9 +1,7 @@
 namespace ScadaLink.Commons.Messages.Management;
 
-public record ListDataConnectionsCommand;
+public record ListDataConnectionsCommand(int? SiteId = null);
 public record GetDataConnectionCommand(int DataConnectionId);
-public record CreateDataConnectionCommand(string Name, string Protocol, string? Configuration);
+public record CreateDataConnectionCommand(int SiteId, string Name, string Protocol, string? Configuration);
 public record UpdateDataConnectionCommand(int DataConnectionId, string Name, string Protocol, string? Configuration);
 public record DeleteDataConnectionCommand(int DataConnectionId);
-public record AssignDataConnectionToSiteCommand(int DataConnectionId, int SiteId);
-public record UnassignDataConnectionFromSiteCommand(int AssignmentId);