Add telegraf_output selector for InfluxDB/Postgres dual-write

Introduces global.telegraf_output (INFLUXDB|POSTGRES|BOTH, default BOTH)
so Telegraf can write metrics to Postgres alongside or instead of
InfluxDB. Each minion authenticates with its own so_telegraf_<minion>
role and writes to a matching schema inside a shared so_telegraf
database, keeping blast radius per-credential to that minion's data.

- Per-minion credentials auto-generated and persisted in postgres/auth.sls
- postgres/telegraf_users.sls reconciles roles/schemas on every apply
- Firewall opens 5432 only to minion hostgroups when Postgres output is active
- Reactor on salt/auth + orch/telegraf_postgres_sync.sls provision new
  minions automatically on key accept
- soup post_to_3.1.0 backfills users for existing minions on upgrade
- so-show-stats prints latest CPU/mem/disk/load per minion for sanity checks
- so-telegraf-trim + nightly cron prune rows older than
  postgres.telegraf.retention_days (default 14)

salt/postgres/soc_postgres.yaml

@@ -0,0 +1,7 @@
+postgres:
+  telegraf:
+    retention_days:
+      description: Number of days of Telegraf metrics to keep in the so_telegraf database. Older rows are deleted nightly by so-telegraf-trim.
+      forcedType: int
+      advanced: True
+      helpLink: influxdb