[Observability] Log search: gateway endpoint and dashboard panel

### Feature Summary
Exported logs cannot be searched from the dashboard. This adds the first complete signal: log search through the gateway plus a UI panel to drive it.
### Proposed Solution or Design
- Backend: a logs query builder and response parsers (`parseLogs`, `parseLogSources`) that apply per-field redaction; a `LogsResource` exposing log search and a source-facets endpoint, registered in `TexeraWebApplication`.
- Frontend: a logs panel with time-window, service, workflow, computing-unit, execution, level, and free-text filters; server-side paging; and sources-backed autofill.
### Affected Area
Workflow UI, Other
### Related
Part of #4070. Depends on #5370.