Observability

Metrics and tracing for the FCA support agent:

• Prometheus + Langfuse Observability — prometheus-fastapi-instrumentator mounted before app.startup, Langfuse keys sourced from Settings
• Prometheus Instrumentator (HTTP Metrics) — should_group_status_codes=False, excluded_handlers for /metrics / /health
• Global Exception Handler — @app.exception_handler(Exception) returning consistent JSON with debug/prod messaging