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

modules/nixos/temp/services/arrtrix/default.nix

@@ -48,6 +48,11 @@
         time_format = " ";
       };
     };
+    observability = {
+      otlp_grpc_endpoint = "";
+      service_name = "arrtrix";
+      resource_attributes = {};
+    };
   };
 in {
   options.services.arrtrix = {