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>

infra/docker-compose.yml

@@ -134,6 +134,10 @@ services:
       - BITBUCKET_API_URL=${BITBUCKET_API_URL:-https://api.bitbucket.org/2.0}
       - BITBUCKET_BASE_URL=${BITBUCKET_BASE_URL:-https://bitbucket.org}
       - GITEA_BASE_URL=${GITEA_BASE_URL:-https://gitea.example.com}
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    group_add:
+      - "${DOCKER_GID:-0}"
     networks:
       - containr-network
     depends_on: