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.ConfigurationDatabase/ScadaLinkDbContext.cs

@@ -35,7 +35,6 @@ public class ScadaLinkDbContext : DbContext, IDataProtectionKeyContext
     // Sites
     public DbSet<Site> Sites => Set<Site>();
     public DbSet<DataConnection> DataConnections => Set<DataConnection>();
-    public DbSet<SiteDataConnectionAssignment> SiteDataConnectionAssignments => Set<SiteDataConnectionAssignment>();
 
     // Deployment
     public DbSet<DeploymentRecord> DeploymentRecords => Set<DeploymentRecord>();