refactor: remove broken NUT/UPS plugin and managed-NUT startup

The UPS plugin's default driver was snmp-ups, so NUT was just translating
network SNMP back into its own protocol — no value over polling the UPS
directly through the existing snmp plugin. The managed-NUT entrypoint path
also broke container startup when config was incomplete.

Deletes nut_setup.py, the NUT block from entrypoint.sh, NUT packages from
the Dockerfile, the ups entry from the plugin catalog example, and the
ups wiring in alerts METRIC_CATALOG, widgets, settings routes, and the
rules form. The plugins/ups source tree (untracked) is also removed from
the working copy. Existing ups_* DB tables are orphaned but harmless in
dev; a drop migration isn't needed.

Follow-up: rebuild "on battery → Ansible shutdown" automation on top of
the alerts system as a new action type that runs a playbook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roundtable/templates/alerts/rules_form.html

@@ -109,9 +109,6 @@
             "unifi":   [("is_up", "1.0 = WAN up, 0.0 = down"),
                         ("latency_ms", "WAN latency in ms"),
                         ("total_clients", "number of connected clients")],
-            "ups":     [("battery_charge_pct", "battery charge 0–100"),
-                        ("on_battery", "1.0 when running on battery"),
-                        ("battery_runtime_secs", "estimated runtime remaining in seconds")],
             "docker":  [("cpu_pct", "container CPU usage %"),
                         ("mem_pct", "container memory usage %")],
             "http":    [("is_up", "1.0 = check passed, 0.0 = failed"),