[codex] Allow opt-in self-hosted registration

[swhan0329]

Summary

Adds an opt-in self-hosted registration path for the unchecked “Enable the option to register new users” item from #1413.

Self-hosted instances still keep the existing safe default: once an owner exists, /register redirects back to login and server-side sign-up is blocked. When an operator explicitly sets DOKPLOY_ALLOW_REGISTRATION=true, the login page exposes the registration link, /register remains reachable, and non-invited email sign-ups can create their own default organization.

Details

• Adds DOKPLOY_ALLOW_REGISTRATION, defaulting to disabled unless the env value is exactly true.
• Keeps first-owner setup behavior unchanged for fresh self-hosted installs.
• Keeps invitation and SSO flows separate from the new open-registration path.
• Guards invited self-hosted signups so the opt-in open-registration path does not create an extra default owner organization for invitation-based onboarding.
• Adds the env var to local and production env examples.
• Adds focused unit coverage for the opt-in flag parser and the default-organization creation decision.

Validation

• pnpm exec biome check --write --no-errors-on-unmatched --files-ignore-unknown=true packages/server/src/constants/index.ts packages/server/src/lib/auth.ts apps/dokploy/__test__/auth/registration-config.test.ts
• pnpm --filter=dokploy exec vitest run __test__/auth/registration-config.test.ts --config __test__/vitest.config.ts
• pnpm --filter=dokploy run typecheck
• pnpm --filter=@dokploy/server run typecheck

Note: local validation ran on Node v20.18.1, so pnpm emitted the repository engine warning for Node ^24.4.0, but the listed checks completed successfully.

/claim #1413