Add OpenTelemetry observability to Arrtrix

- Add OTLP/gRPC observability config and resource attributes
- Instrument webhook and onboarding handlers with tracing and metrics
- Add OpenTelemetry dependencies to go.mod/go.sum
- Update NixOS modules to configure observability settings

packages/arrtrix/pkg/runtime/example.go

@@ -56,6 +56,13 @@ logging:
     - type: stdout
       format: pretty-colored
 
+observability:
+  # OTLP/gRPC endpoint for logs, traces, and metrics.
+  # Set to e.g. http://127.0.0.1:4317 to enable export.
+  otlp_grpc_endpoint: ""
+  service_name: arrtrix
+  resource_attributes: {}
+
 management_room_texts:
   welcome: ""
   welcome_connected: ""