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/constants.js

@@ -5,7 +5,7 @@
 // ── App Identity ──────────────────────────────────────────────
 const APP = {
   NAME: 'DashCaddy',
-  VERSION: '1.0',
+  VERSION: '1.1',
   PORT: 3001,
   USER_AGENTS: {
     PROBE:       'DashCaddy-Probe/1.0',