OpenTelemetry distributed tracing

Set OTEL_EXPORTER_OTLP_ENDPOINT to activate. Instrumentation covers FastAPI routes, asyncpg connection pool queries, and outbound httpx calls (LLM waterfall, webhook dispatch). The dashboard Core API client propagates a W3C traceparent header so traces stitch end-to-end from UI interaction to policy evaluation. Falls back to NoOp with zero overhead when the env var is unset.