fix(observability): use schemaless resource attrs to avoid schema URL conflict

SetupTracing merged resource.Default() (schema 1.43.0 under otel 1.45.0)
with semconv/v1.24.0's schema URL, so resource.Merge errored with
'conflicting Schema URL' and the backend exited at startup whenever an
OTLP endpoint was configured. resource.NewSchemaless keeps service.name
without pinning a schema, surviving otel major bumps.

backend/internal/observability/observability.go

@@ -77,10 +77,12 @@ func SetupTracing(ctx context.Context, cfg Config) (func(context.Context) error,
 	if err != nil {
 		return nil, err
 	}
+	// Schemaless: a non-empty but different schema URL here (from an older
+	// semconv pin) makes resource.Merge fail as soon as the SDK's
+	// resource.Default() schema URL changes across otel upgrades.
 	res, err := resource.Merge(
 		resource.Default(),
-		resource.NewWithAttributes(
-			semconv.SchemaURL,
+		resource.NewSchemaless(
 			semconv.ServiceName(name),
 		),
 	)