feat: complete OpenTelemetry distributed tracing integration

[prismn]

Summary

Completes the OpenTelemetry distributed tracing implementation across all services as specified in issue #75.

Implementation Details

src/config/tracing.ts

• Added Jaeger exporter alongside the existing OTLP exporter (activated via JAEGER_AGENT_HOST or JAEGER_ENDPOINT env vars — falls back to OTLP-native Jaeger 1.41+)
• Added W3CTraceContextPropagator so trace context propagates correctly across service boundaries via traceparent/tracestate HTTP headers
• Added extractContext and injectContext helpers for explicit context propagation at service boundaries
• Replaced deprecated spanProcessor with spanProcessors array to support multiple exporters
• Guarded shutdownTracing against being called before SDK is started

src/observability/tracing.interceptor.ts (new)

• TracingInterceptor — NestJS interceptor that wraps every HTTP handler in a named OTel span
• Extracts upstream trace context from incoming headers (distributed tracing across microservices)
• Records http.method, http.url, http.route, handler.class, handler.method, and http.status_code
• Records exceptions and sets ERROR span status on failures

src/observability/observability.module.ts

• Registers TracingInterceptor globally via APP_INTERCEPTOR so 100% of HTTP endpoints are traced

src/dashboard/websocket/dashboard.gateway.ts

• Adds OTel spans for WebSocket connect and disconnect events with client ID, namespace, and trace context attributes

Auto-instrumentation coverage (via @opentelemetry/auto-instrumentations-node)

• Database queries — TypeORM/pg instrumented with timing metrics by the HTTP auto-instrumentation
• External AI provider / HTTP calls — all outgoing HTTP requests instrumented automatically
• HTTP servers — incoming request headers (x-request-id, x-correlation-id) captured in span attributes

Validation Results

• ✅ npm run build — compiled successfully (webpack)
• ⚠️ npm test — pre-existing RangeError: Maximum call stack size exceeded in @nestjs/core injector; not introduced by this change (confirmed identical failure on upstream main before changes)

Closes #75