Add opt-in Xquik posting backend

[kriptoburak]

Summary

• add TWITTER_BACKEND=xquik as an optional posting backend
• keep Tweepy as the default backend for CLI, web UI, and direct scripts
• add unit tests for Xquik configuration and accepted writes

Validation

• python3 -m unittest -v tests/test_xquik_client.py
• python3 -m compileall cli.py web.py src tests
• git diff --check

[lewispour]

Xquik appears to be a subscription based API with paid pricing, I cannot test the functionality without a paid account.

[kriptoburak]

Thanks for checking. You do not need Xquik credentials to verify the unchanged default flow: Tweepy remains the default backend, and the Xquik path is covered by mocked unit tests. It is opt-in only for users who set TWITTER_BACKEND=xquik with their own connected account. Happy to keep it out if you prefer this project to stay limited to direct X credentials.