fix(deployment): instance delete fully removes the record

Deleting an instance only undeployed it from the site and set the state
to NotDeployed, leaving an orphan record that could never be removed —
the state-transition matrix rejected delete from NotDeployed.

Delete now removes the instance record entirely (deployment history,
snapshot, attribute/alarm overrides, and connection bindings go with
it), and is permitted from any state.

tests/ScadaLink.DeploymentManager.Tests/DeploymentServiceTests.cs

@@ -188,15 +188,14 @@ public class DeploymentServiceTests
     }
 
     [Fact]
-    public async Task DeleteInstanceAsync_WhenNotDeployed_ReturnsTransitionError()
+    public async Task DeleteInstanceAsync_InstanceNotFound_ReturnsFailure()
     {
-        var instance = new Instance("TestInst") { Id = 1, SiteId = 1, State = InstanceState.NotDeployed };
-        _repo.GetInstanceByIdAsync(1).Returns(instance);
+        _repo.GetInstanceByIdAsync(1).Returns((Instance?)null);
 
         var result = await _service.DeleteInstanceAsync(1, "admin");
 
         Assert.True(result.IsFailure);
-        Assert.Contains("not allowed", result.Error);
+        Assert.Contains("not found", result.Error);
     }
 
     // ── WP-8: Deployment comparison ──