chore: remove pgpm-cron module and scheduled_cron table

[pyramation]

Summary

Removes the dead pg_cron machinery that #95 upstreamed: maintenance scheduling was consolidated under app_jobs.scheduled_jobs (constructive-io/constructive-db#2012), so pgpm_cron.sync_scheduled_cron() and the metaschema_public.scheduled_cron registry table have no consumers.

• Deletes packages/pgpm-cron entirely (module, plan, compiled sql).
• Drops schemas/metaschema_public/tables/scheduled_cron/table from metaschema-schema (plan line + deploy/revert/verify) and regenerates sql/metaschema-schema--0.15.5.sql via pgpm package.
• Removes the pgpm-cron entry from MODULES.md.

Link to Devin session: https://app.devin.ai/sessions/7d74797f980143949a673f4bfff58ebe
Requested by: @pyramation

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