Adopt pg_partman + pg_cron for Telegraf metric tables

Every telegraf.* metric table is now a daily time-range partitioned
parent managed by pg_partman. Retention drops old partitions instead
of the row-by-row DELETE that so-telegraf-trim used to run nightly,
and dashboards will benefit from partition pruning at query time.

- Load pg_cron at server start via shared_preload_libraries and point
  cron.database_name at so_telegraf so job metadata lives alongside
  the metrics
- Telegraf create_templates override makes every new metric table a
  PARTITION BY RANGE (time) parent registered with partman.create_parent
  in one transaction (1 day interval, 3 premade)
- postgres_telegraf_group_role now also creates pg_partman and pg_cron
  extensions and schedules hourly partman.run_maintenance_proc
- New retention reconcile state updates partman.part_config.retention
  from postgres.telegraf.retention_days on every apply
- so_telegraf_trim cron is now unconditionally absent; script stays on
  disk as a manual fallback

salt/postgres/defaults.yaml

@@ -14,3 +14,5 @@ postgres:
     log_destination: 'stderr'
     logging_collector: 'off'
     log_min_messages: 'warning'
+    shared_preload_libraries: pg_cron
+    cron.database_name: so_telegraf