Add Xquik scheduler backend

[kriptoburak]

Summary

• Add an opt-in SCHEDULER_POSTING_BACKEND=xquik path for text-only scheduler posts.
• Keep Twitter OAuth posting as the default path and keep media or community posts on the existing Twitter backend.
• Document the Xquik env vars and preserve existing setup.

Validation

• Installed dependencies with lifecycle scripts disabled for static checks.
• Ran the cron TypeScript check against tsconfig.cron.json.
• Ran git diff whitespace check.

Notes

• A normal npm ci is blocked locally because better-sqlite3@9.6.0 has no prebuilt binary for Node v26.0.0 and the native rebuild fails on the local compiler standard.
• npm run lint opens Next.js ESLint setup because no ESLint config is present.