feat: upstream pgpm-cron module + metaschema_public.scheduled_cron table

[pyramation]

Summary

Upstreams the pg_cron work from constructive-db#1988 so this repo stays the source of truth for shared modules:

• packages/metaschema-schema: new metaschema_public.scheduled_cron table (declarative pg_cron job registry — database_id FK cascade, name, schedule, command, is_enabled, unique on (database_id, name)), plus plan entry and regenerated sql/metaschema-schema--0.15.5.sql bundle.
• packages/pgpm-cron (new module, @pgpm/cron): open-code sync mechanics — pgpm_cron.sync_scheduled_cron() reconciles scheduled_cron rows into pg_cron (cron.schedule for enabled rows, guarded cron.unschedule for disabled ones; silently no-ops when pg_cron is absent). Intended for local/exported single-database deployments; the platform cross-DB schedule_in_database fan-out lives in constructive-db. Cross-requires metaschema-schema:.../scheduled_cron/table.
• Listed pgpm-cron in MODULES.md.

Note: pg_cron is one-per-cluster, so it is deliberately NOT a control-file requires — the module is safe to deploy into databases that will never host pg_cron (see the package README).

Verified locally against postgres-plus:18: pgpm deploy/verify/revert --package pgpm-cron full cycle passes and the scheduled_cron table lands with the expected shape; packages/metaschema-schema jest suite passes.

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

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