* feat(observability): add trace-id correlation and enhanced logging
- add opt-in gateway request trace correlation via X-Trace-Id
- enhance logging with configurable trace_id-aware formatting
- propagate deerflow_trace_id into runtime context and Langfuse metadata
- keep enhanced logging disabled by default to preserve existing behavior
* fix: harden trace correlation wiring
- Make logging enhancement a restart-required startup snapshot and remove per-request config reads from TraceMiddleware
- Restrict trace ids to printable ASCII before writing them to response headers, logs, and Langfuse metadata
- Gate implicit DeerFlowClient trace-id creation behind logging.enhance.enabled while preserving explicit caller opt-in
- Bind embedded client trace context per stream step to avoid generator ContextVar leaks and cross-context reset errors
- Rebind memory update trace ids in Timer/executor worker paths so enhanced logs keep the captured correlation id
- Remove unrelated __run_journal context overwrite from the trace-correlation change set
* fix(gateway): avoid eager app construction on package import
* fix(gateway): avoid config load during app import
Keep Gateway app construction import-safe when config.yaml is absent by
disabling TraceMiddleware only for that construction-time fallback path.
Startup lifespan still performs strict config loading before serving.
