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>

docs/plugins/index.yaml.example

@@ -94,18 +94,3 @@ plugins:
       - network
       - unifi
       - ubiquiti
-
-  - name: ups
-    version: "1.0.0"
-    description: "UPS monitoring via NUT (Network UPS Tools) with Ansible shutdown automation"
-    author: "Roundtable"
-    license: "MIT"
-    min_app_version: "0.1.0"
-    repository_url: "https://git.fabledsword.com/bvandeusen/Roundtable-plugins"
-    homepage: "https://git.fabledsword.com/bvandeusen/Roundtable-plugins/src/branch/main/ups"
-    download_url: "https://git.fabledsword.com/bvandeusen/Roundtable-plugins/releases/download/ups-v1.0.0/ups.zip"
-    checksum_sha256: ""
-    tags:
-      - ups
-      - power
-      - nut