feat(vss): log user_token (JWT sub) on auth and request log/sentry sites

[bumi]

Claude added logging the JWT sub to potentially help with maintenance identifying usage.

Summary

Adds the authenticated user_token (the JWT sub claim) as a structured field on the log and Sentry sites in handle_request, so requests can be correlated to a user/node identifier in app logs, Datadog spans, and Sentry.

Previously user_token was deliberately excluded from all tracing spans (skip(...)) and never appeared in logs or Sentry messages.

Changes (all in rust/server/src/vss_service.rs)

• "Authentication successful" log now includes user_token.
• Request-lifecycle logs/spans carry user_token: body-too-large (413), request-completed (200), key-not-found.
• Error paths include user_token in both the tracing event and the Sentry message: internal server error, generic request error, protobuf parse error.
• The handler call now passes user_token.clone() so the value stays available for the post-handler log/sentry sites (it was previously moved into the handler).