feat(tracing): add OpenTelemetry instrumentation to API gateway

- Add internal/tracing package with OTel tracer initialization
- HTTP middleware for server-side tracing (request/response attributes)
- Transport wrapper for client-side upstream call tracing
- Update proxy to use tracing transport
- Add OTEL_* env vars to k8s deployment

Traces flow: api-gateway -> otel-collector -> tempo -> grafana

cmd/gateway/main.go

@@ -14,9 +14,26 @@ import (
 	"forgejo.riotpiao.com/rock/homelab-frontend/internal/server"
 	"forgejo.riotpiao.com/rock/homelab-frontend/internal/serviceadapter"
 	"forgejo.riotpiao.com/rock/homelab-frontend/internal/temporal"
+	"forgejo.riotpiao.com/rock/homelab-frontend/internal/tracing"
 )
 
 func main() {
+	ctx := context.Background()
+
+	// Initialize OpenTelemetry tracing
+	tracingCfg := tracing.DefaultConfig()
+	shutdownTracer, err := tracing.Init(ctx, tracingCfg)
+	if err != nil {
+		log.Printf("warning: failed to initialize tracing: %v", err)
+	} else {
+		log.Printf("tracing initialized: service=%s endpoint=%s", tracingCfg.ServiceName, tracingCfg.OTLPEndpoint)
+		defer func() {
+			if err := shutdownTracer(ctx); err != nil {
+				log.Printf("error shutting down tracer: %v", err)
+			}
+		}()
+	}
+
 	// Load configuration
 	cfg, err := config.Load()
 	if err != nil {
@@ -63,7 +80,10 @@ func main() {
 	// Create router that handles health endpoints, X-Service (ServiceAdapter) routing,
 	// temporal endpoints, and passes others to upstream handler
 	router := server.NewRouter(healthChecker, dispatcher, temporalHandler, upstreamHandler)
-	srv.SetHandler(router)
+
+	// Wrap router with tracing middleware
+	tracedRouter := tracing.Middleware(router)
+	srv.SetHandler(tracedRouter)
 
 	// Set up signal handling
 	sigChan := make(chan os.Signal, 1)