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>

app/backend/internal/api/auth_proxy.go

@@ -67,7 +67,9 @@ func shouldBlockPublicBetterAuthSignup(c *gin.Context, cfg *config.Config, db *d
 		return true
 	}
 
-	return count > 0
+	// Registration is closed once the first account exists unless the owner
+	// reopens it in Settings (signup_enabled).
+	return count > 0 && !signupEnabled(db)
 }
 
 func parseAuthProxyTarget(rawTarget string) (*url.URL, error) {