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

k8s/deployment.yaml

@@ -61,6 +61,15 @@ spec:
           value: "5m"
         - name: LOG_LEVEL
           value: "info"
+        # OpenTelemetry tracing configuration
+        - name: OTEL_EXPORTER_OTLP_ENDPOINT
+          value: "otel-collector.tracing.svc.cluster.local:4317"
+        - name: OTEL_SERVICE_NAME
+          value: "api-gateway"
+        - name: OTEL_SERVICE_VERSION
+          value: "1.0.0"
+        - name: OTEL_ENVIRONMENT
+          value: "production"
         volumeMounts:
         - name: config
           mountPath: /etc/gateway