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.CentralUI/Components/Pages/Deployment/Instances.razor

@@ -697,13 +697,8 @@
         var attrs = await TemplateEngineRepository.GetAttributesByTemplateIdAsync(inst.TemplateId);
         _bindingDataSourceAttrs = attrs.Where(a => !string.IsNullOrEmpty(a.DataSourceReference)).ToList();
 
-        // Load data connections for this site
+        // Load data connections for this site (each connection now belongs to exactly one site)
         _siteConnections = (await SiteRepository.GetDataConnectionsBySiteIdAsync(inst.SiteId)).ToList();
-        if (_siteConnections.Count == 0)
-        {
-            // Also show unassigned connections (they may not be assigned to a site yet)
-            _siteConnections = (await SiteRepository.GetAllDataConnectionsAsync()).ToList();
-        }
 
         // Load existing bindings
         var existingBindings = await TemplateEngineRepository.GetBindingsByInstanceIdAsync(inst.Id);