fan-control: presence-aware IPMI fan curve for the R730 PVE host

The iDRAC stock curve runs the CPU at ~72°C on the 7080 RPM floor even
under load (optimises for quiet, not cool). Add a bash daemon + systemd
unit that drives the chassis fans from CPU temp on two curves, picked by
garage occupancy (the server is in the garage): COOL when empty
(measured ~58-65°C under load), QUIET near the silent floor when the
ha-sofia garage door shows someone is there (open, or <15min since last
activity).

Manual fan mode is backstopped: bash EXIT trap + systemd ExecStopPost
hand fans back to Dell auto on stop/crash; CPU>=83°C or repeated IPMI
failures do the same. Pushgateway metrics (job=fan_control). 36 unit
tests cover the pure curve/hysteresis/presence/parse logic; DRY_RUN +
RUN_ONCE for integration checks. Deployed and verified on 192.168.1.127
(CPU 70->58°C in cool mode, hysteresis stepping confirmed).

Design:  docs/plans/2026-06-04-pve-fan-control-design.md
Runbook: docs/runbooks/fan-control.md

[ci skip]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/fan-control.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Presence-aware IPMI fan controller (Dell R730, garage)
+Documentation=https://github.com/ViktorBarzin/infra/blob/master/scripts/fan-control.sh
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+EnvironmentFile=-/etc/fan-control.env
+ExecStart=/usr/local/bin/fan-control
+# Belt-and-suspenders: whatever happens to the daemon, hand the fans back to
+# the iDRAC's own automatic curve so the box is never stuck in manual mode.
+ExecStopPost=/usr/bin/ipmitool raw 0x30 0x30 0x01 0x01
+Restart=on-failure
+RestartSec=10
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=fan-control
+
+[Install]
+WantedBy=multi-user.target