Add per-span API endpoint for span details (including properties)

The GET /api/v1/runs/{runId}/trace response does not include data.properties (or resourceProperties) on spans when using the ClickHouse event store, and we don’t want to add them to the full trace response because that could make the payload very large. Add a public per-span endpoint, e.g. GET /api/v1/runs/{runId}/spans/{spanId}, that returns a single span with full details (including properties and resourceProperties), using the same data the dashboard loads when you open a span. API consumers can then call /trace for the tree and optionally call the per-span endpoint only for the spans they need, keeping the default trace response small.