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>

Dockerfile

@@ -3,9 +3,6 @@ FROM python:3.13-slim
 RUN DEBIAN_FRONTEND=noninteractive apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         iputils-ping \
-        # NUT (Network UPS Tools) — only used when NUT_MANAGED=1
-        nut \
-        nut-client \
         # gosu — minimal privilege-drop tool used by entrypoint.sh
         gosu \
     && rm -rf /var/lib/apt/lists/*
@@ -23,10 +20,9 @@ COPY alembic.ini .
 COPY entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
 
-# Runtime directories — owned by app user.
-# The container runs as root so the entrypoint can start NUT daemons (USB
-# access requires root); it drops to 'app' (uid 1000) via gosu before
-# launching roundtable.
+# Runtime directories — owned by app user. The container starts as root
+# so the entrypoint can fix up /data permissions if needed, then drops to
+# 'app' (uid 1000) via gosu before launching roundtable.
 RUN useradd -m -u 1000 app \
     && mkdir -p /data/playbook_cache /data/plugins \
     && chown -R app:app /data /app