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/vars/globals.map.jinja

@@ -24,6 +24,7 @@
     'md_engine': INIT.PILLAR.global.mdengine,
     'pcap_engine': GLOBALMERGED.pcapengine,
     'pipeline': GLOBALMERGED.pipeline,
+    'telegraf_output': GLOBALMERGED.telegraf_output,
     'so_version': INIT.PILLAR.global.soversion,
     'so_docker_gateway': DOCKERMERGED.gateway,
     'so_docker_range': DOCKERMERGED.range,