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.go

@@ -29,6 +29,8 @@ import (
 	"go.opentelemetry.io/otel/sdk/resource"
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	"go.opentelemetry.io/otel/trace"
+
+	"scrabble/pkg/version"
 )
 
 // Exporter selectors supported per signal.
@@ -95,9 +97,7 @@ func New(ctx context.Context, cfg Config) (*Runtime, error) {
 		return nil, err
 	}
 
-	res, err := resource.New(ctx, resource.WithAttributes(
-		attribute.String("service.name", cfg.ServiceName),
-	))
+	res, err := serviceResource(ctx, cfg)
 	if err != nil {
 		return nil, fmt.Errorf("telemetry: build resource: %w", err)
 	}
@@ -122,6 +122,16 @@ func New(ctx context.Context, cfg Config) (*Runtime, error) {
 	return &Runtime{tracerProvider: tracerProvider, meterProvider: meterProvider}, nil
 }
 
+// serviceResource builds the OpenTelemetry resource describing this service: its
+// service.name and the service.version stamped into the binary at build time
+// (pkg/version, set from the git tag by the deploy).
+func serviceResource(ctx context.Context, cfg Config) (*resource.Resource, error) {
+	return resource.New(ctx, resource.WithAttributes(
+		attribute.String("service.name", cfg.ServiceName),
+		attribute.String("service.version", version.Version),
+	))
+}
+
 // TracerProvider returns the runtime tracer provider, or the global one when r is
 // not initialised.
 func (r *Runtime) TracerProvider() trace.TracerProvider {