Add auto-update system for DashCaddy instances

- self-updater.js: polls for new versions, downloads/verifies tarballs,
  triggers host-side rebuild via systemd path unit
- dashcaddy-update.sh + systemd units: host-side container rebuild with
  automatic rollback on health check failure
- 7 new /api/v1/system/* endpoints for version info, update check/apply,
  rollback, and update history
- Frontend: DashCaddy tab in Updates modal with version display,
  changelog, update button, rollback, and notification dot
- install.sh: updater service installation, volume mounts, env vars
- build-release.sh + webhook-handler.js: release server pipeline
  (Gitea webhook → build tarball → deploy to get.dashcaddy.net)
- Dockerfile: DASHCADDY_COMMIT build arg → VERSION file
- Version bump to 1.1.0

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

dashcaddy-api/scripts/dashcaddy-updater.path

@@ -0,0 +1,10 @@
+[Unit]
+Description=Watch for DashCaddy update trigger
+Documentation=https://dashcaddy.net
+
+[Path]
+PathChanged=/opt/dashcaddy/updates/trigger.json
+MakeDirectory=yes
+
+[Install]
+WantedBy=multi-user.target