Add optional Xquik posting backend

[kriptoburak]

Summary

• Add TWITTER_BACKEND=xquik as an opt-in posting backend while keeping x-api as the default.
• Route CLI, dashboard posting, and scheduled posting through the selected backend.
• Post text tweets through Xquik's /api/v1/x/tweets endpoint with account, text, and reply IDs.
• Keep local media uploads on the existing X API backend because Xquik media uses public URLs.
• Document the Xquik env vars and add tests for Xquik 200 responses, pending confirmations, and local media rejection.

Validation

• bun test
• bun run typecheck
• git diff --check
• Source checked against https://xquik.com/openapi.json for /api/v1/x/tweets request and response shapes.