feat(company-codex): add managed GUI access and audit

[hongzexin]

Summary

• add self-service managed Codex GUI credentials with CC Switch and official GUI setup
• verify membership on every gateway request and retain synchronized Responses sessions for workspace audit
• revoke upstream access during member/workspace removal and add FK-free concurrent-index migrations

Verification

• cd server && go test ./internal/handler ./cmd/server
• pnpm --filter @multica/core typecheck
• pnpm --filter @multica/views typecheck
• pnpm --filter @multica/core exec vitest run api/schemas.test.ts
• pnpm --filter @multica/views exec vitest run locales/parity.test.ts settings/components/settings-page.test.tsx
• package-scoped ESLint for changed core/views files