feat: stamp the build version into every service

pkg/version.Version (default "dev") is set at link time via -ldflags from each
service Dockerfile's VERSION build-arg, which the deploy passes as the git tag
(git describe --tags). It surfaces as the OpenTelemetry service.version resource
attribute (so Grafana/Tempo are version-aware), alongside the SPA's existing
About version. Adds the VERSION build-arg to the backend/gateway/validator/bot
compose builds and a serviceResource test covering service.name + service.version.

pkg/telemetry/telemetry_test.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"testing"
 	"time"
+
+	"scrabble/pkg/version"
 )
 
 // TestConfigValidate covers the supported and rejected exporter selections.
@@ -82,3 +84,22 @@ func TestNilRuntime(t *testing.T) {
 		t.Errorf("nil runtime Shutdown: %v", err)
 	}
 }
+
+// TestServiceResource checks the resource carries service.name and the embedded
+// service.version (pkg/version, stamped at build time).
+func TestServiceResource(t *testing.T) {
+	res, err := serviceResource(context.Background(), DefaultConfig("svc"))
+	if err != nil {
+		t.Fatalf("serviceResource: %v", err)
+	}
+	attrs := map[string]string{}
+	for _, kv := range res.Attributes() {
+		attrs[string(kv.Key)] = kv.Value.AsString()
+	}
+	if attrs["service.name"] != "svc" {
+		t.Errorf("service.name = %q, want svc", attrs["service.name"])
+	}
+	if attrs["service.version"] != version.Version {
+		t.Errorf("service.version = %q, want %q", attrs["service.version"], version.Version)
+	}
+}