fix(cluster-1): data corruption fixes (C4/C5/C8)

- synchandler: combine course date with clock time via time.Date instead
  of adding two absolute Unix epochs, which produced corrupt start times
- tracing: make DeploymentEnvironment configurable via config.Trace
  (defaults to development instead of hardcoded production)
- http: align course handler tracer name to 'kuriweb.http' to match the
  request middleware instrument so spans share the same tracer

cmd/kuriweb/trace.go

@@ -50,7 +50,7 @@ func setupOtelSDK(ctx context.Context, cfg config.Trace) (shutdown shutdownFunc,
 		return err
 	}
 
-	resource, err := makeServiceResource(ctx)
+	resource, err := makeServiceResource(ctx, cfg.Environment)
 	if err != nil {
 		return shutdown, fmt.Errorf("making service resource: %w", err)
 	}
@@ -102,7 +102,10 @@ type TraceProviderParams struct {
 	Type       config.TraceClientType
 }
 
-func makeServiceResource(ctx context.Context) (*resource.Resource, error) {
+func makeServiceResource(ctx context.Context, environment string) (*resource.Resource, error) {
+	if environment == "" {
+		environment = "development"
+	}
 	r, err := resource.New(
 		ctx,
 		resource.WithDetectors(
@@ -113,7 +116,7 @@ func makeServiceResource(ctx context.Context) (*resource.Resource, error) {
     resource.WithHost(),
 		resource.WithAttributes(
 			semconv.ServiceName("bigstats:kuriweb"),
-			semconv.DeploymentEnvironment("production"),
+			semconv.DeploymentEnvironment(environment),
 		),
 	)
 	if err != nil {