feat(hub): support optional OpenTelemetry server tracing #34

Self-hosted Hub operators need to send server-side telemetry to their own APM without rebuilding the published image or requiring a vendor-specific SDK. Add opt-in OpenTelemetry tracing configured with standard OTEL_* environment variables, covering Rails requests, background jobs, and supported database and outbound HTTP calls. Export is disabled by default and uses OTLP over HTTP/protobuf. Keep credentials, request bodies, job arguments, SQL parameter values, and sensitive URLs out of exported telemetry. Provider outages must not block Hub requests. Configuration changes require restarting web and job processes. Document direct export and company-managed Collector deployment. Browser monitoring, log/metric export, bundled Collectors, admin settings pages, and vendor-native error-reporting features are outside this first version.