Postgres partitioning enhancements: sub-partitioning + tablespace on partitioned parents

[ihistand]

Remaining native-partitioning gaps (cli/api/dbadapters/postgres_execution_sql.ts createPartitionedTableTasks):

• Sub-partitioning (nested PARTITION BY) — not supported.
• Tablespace on the partitioned parent — createPartitionedTableTasks omits it (the regular createTable applies it). Minor.

Not gaps (confirmed): indexes + opclass on partitioned tables already work (createIndexes runs for the partitioned path). fillfactor on a partitioned parent is invalid — Postgres forbids storage params on partitioned tables. Split from #15.