- Removed OpenTelemetry (TracerProvider, OTLPSpanExporter, LoggingInstrumentor, FastAPIInstrumentor)
- Implemented Langfuse v3.11.0 with CallbackHandler for LLM tracing
- Added langfuse_trace_context() with propagate_attributes() for proper user_id filtering
- Fixed user_id to appear in Langfuse User ID filter (not just metadata)
- Added session_id and tags propagation for trace organization
- Updated controller.py to wrap graph execution in langfuse_trace_context
- Verified traces send to self-hosted Langfuse at http://172.16.2.207:3009
- Configuration: LANGFUSE_PUBLIC_KEY, LANGFUSE_SECRET_KEY, LANGFUSE_BASE_URL in .env

GIAI_DOAN_1 COMPLETE: LLM observability via Langfuse SDK
Next: GIAI_DOAN_2 - Container monitoring stack (cAdvisor + Prometheus + Grafana)