chore(persistence): Implement row TTL with pg_partman

[arunderwood]

Summary

Implement database-level partitioning and automatic partition maintenance for spots table. Replace application-level @scheduled cleanup when pg_partman available on Render.

Context

Currently using application-level cleanup. Database partitioning is more efficient for high-volume time-series data.

Deferred

This is tech debt to address when pg_partman is available on Render.