Add deploy_check marker to /api/v1/health for CD verification

Temporary canary field to confirm the staging pipeline deploys what
lands on the branch. Verify with:
curl https://api-stg-balsm-health.mosalam.com/api/v1/health | grep cd-canary

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Hshqj5eFfJNWCX6XNMxCh

Author: hossameldinmi

src/Balsm.API/Controllers/HealthController.cs

@@ -20,7 +20,8 @@ public IActionResult Get()
             ready = gate.IsReady,
             version = typeof(HealthController).Assembly.GetName().Version?.ToString() ?? "0.0.0",
             uptime_seconds = uptimeSeconds,
-            not_ready_reason = gate.IsReady ? null : gate.Reason
+            not_ready_reason = gate.IsReady ? null : gate.Reason,
+            deploy_check = "cd-canary-2026-07-06"
         });
     }
 }

tests/Balsm.API.Tests/Controllers/HealthControllerTests.cs

@@ -28,6 +28,19 @@ public void Get_ReturnsOkWithHealthyStatus()
         statusProperty.Should().NotBeNull();
     }
 
+    [Fact]
+    public void Get_ReturnsDeployCheckMarker()
+    {
+        var controller = CreateController();
+
+        var result = controller.Get();
+
+        var okResult = result.Should().BeOfType<OkObjectResult>().Subject;
+        var value = okResult.Value;
+        var marker = value!.GetType().GetProperty("deploy_check")?.GetValue(value);
+        marker.Should().Be("cd-canary-2026-07-06");
+    }
+
     [Fact]
     public void Get_ReturnsTimestamp()
     {