Track pipelines_id_range on ci_partitions

What does this MR do and why?

Track pipelines_id_range on ci_partitions

Adds pipelines_id_range int8range column to ci_partitions with a GiST exclusion constraint that prevents overlapping ranges. The new column is populated by Ci::Partitions::SyncService when switching writes, capturing MIN/MAX(commit_id) from the outgoing partition and assigning an open-ended range to the incoming one. A post-deploy migration backfills existing partitions using MIN(commit_id) from p_ci_builds, clamped to the next partition's MIN(commit_id) to avoid GiST overlap.

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Related to #593701 (closed)