[Observability] PR7: traces end-to-end (gateway + UI panel)

[Ma77Ball]

PR7: Traces end-to-end

Distributed-trace retrieval through the gateway plus a trace-tree UI panel.

Backend

• Trace query builder and parseTraces.
• TracesResource exposing trace retrieval by id, with id validation and scope enforcement at the gateway.

Frontend

• Traces panel rendering spans as a tree built from parent-span relationships, with pivot-from-log so a log row can open its trace directly. Span fields are bound with text interpolation only.

Part of #4070. Stacked on PR6 (#5372).
Branch: obs/pr7/traces.

[Ma77Ball]

/take

[Ma77Ball]

/parent-issue #4070