Investigate migrating JobAnalytics from ci_finished_builds to siphon_p_ci_builds

Context

p_ci_builds is now fully replicated to ClickHouse via Siphon as siphon_p_ci_builds, with traversal_path populated through the project_traversal_paths_dict dictionary.

Today, JobAnalytics (project GraphQL field powering the Jobs panel on /-/pipelines/charts) reads from the ci_finished_builds ReplacingMergeTree table joined with ci_finished_pipelines (for source/ref/date filtering of the parent pipeline). Both tables are CSV-synced from Postgres by dedicated sync services.

This issue tracks a feasibility analysis and (if green-lit) execution of switching the data source to Siphon-synced tables (siphon_p_ci_builds + siphon_p_ci_pipelines + siphon_p_ci_stages for stage_name), so we can retire the bespoke sync pipelines. (gitlab.com first)

Related: #598440 (closed)