fix(host): resolve Host-003,004 — replace plaintext secrets with env placeholders, validate site seed-node ports; re-triage Host-002

tests/ScadaLink.Host.Tests/HostStartupTests.cs

@@ -28,6 +28,8 @@ public class HostStartupTests : IDisposable
         // Set the environment to Central so appsettings.Central.json is loaded,
         // and set DOTNET_ENVIRONMENT before the factory creates the host.
         var previousEnv = Environment.GetEnvironmentVariable("DOTNET_ENVIRONMENT");
+        // Host-003: connection strings are externalised; supply them via env vars.
+        using var dbEnv = new CentralDbTestEnvironment();
         try
         {
             Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", "Central");