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/alerts/routes.py

@@ -20,7 +20,6 @@ METRIC_CATALOG: dict[str, list[str]] = {
     "traefik": ["request_rate", "error_rate", "latency_p50_ms", "latency_p95_ms",
                 "latency_p99_ms", "response_bytes_rate", "cert_expiry_days"],
     "unifi":   ["is_up", "latency_ms", "total_clients"],
-    "ups":     ["battery_charge_pct", "on_battery", "battery_runtime_secs"],
     "docker":  ["cpu_pct", "mem_pct"],
     "http":    ["is_up", "response_ms"],
     # snmp metrics are user-defined OID labels — populated dynamically from plugin_metrics