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/core/widgets.py

@@ -162,38 +162,6 @@ WIDGET_REGISTRY: dict[str, dict] = {
             },
         ],
     },
-    "ups_status": {
-        "key": "ups_status",
-        "label": "UPS — Status",
-        "description": "Battery charge, runtime estimate, load percentage, and on-battery alerts",
-        "hx_url": "/plugins/ups/widget",
-        "detail_url": "/plugins/ups/",
-        "plugin": "ups",
-        "poll": True,
-        "params": [],
-    },
-    "ups_history": {
-        "key": "ups_history",
-        "label": "UPS — History Chart",
-        "description": "Battery %, load %, and input voltage over time (Chart.js)",
-        "hx_url": "/plugins/ups/widget/history",
-        "detail_url": "/plugins/ups/",
-        "plugin": "ups",
-        "poll": True,
-        "params": [
-            {
-                "key": "hours",
-                "label": "Time range",
-                "type": "select",
-                "default": 6,
-                "options": [
-                    {"value": 1,  "label": "Last 1 hour"},
-                    {"value": 6,  "label": "Last 6 hours"},
-                    {"value": 24, "label": "Last 24 hours"},
-                ],
-            },
-        ],
-    },
     "docker_containers": {
         "key": "docker_containers",
         "label": "Docker — Containers",