feat: owner accounts and in-app platform settings

- First registered account becomes admin (users.is_admin, backfilled
  to earliest account); better-auth mirror path does the same
- Public registration closes after bootstrap; admin can reopen it via
  GET/PUT /api/v1/settings (admin-gated) or SIGNUP_ENABLED env
- In-app settings override env vars; secrets stored in app_settings,
  token values never returned by the API
- Saving a Cloudflare Tunnel token pulls and runs a managed
  cloudflared sidecar on the backend's networks; clearing removes it;
  startup restores it from saved settings
- group_add DOCKER_GID grants the non-root backend user Docker socket
  access; install.sh probes the in-container socket gid
- Settings page gains an owner-only Platform section (registration
  toggle, tunnel token, link to Cloudflare dashboard); sign-up page
  stays reachable while registration is open
- OpenAPI spec + generated types updated; manual user creation now
  requires admin

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

README.md

@@ -61,7 +61,8 @@ Installs into `./containr`, generates secrets, pulls the published `:latest`
 images, and starts the stack — UI at `http://localhost:3000`, API at
 `http://localhost:8082`. Non-interactive; re-runs never overwrite `.env`.
 On first visit you create the owner account (email/password); registration
-then closes automatically.
+then closes automatically. The owner can reopen it and configure a Cloudflare
+Tunnel token under **Settings → Platform** — in-app values override env vars.
 
 Override with env vars: