Add optional Xquik posting backend

[kriptoburak]

Summary

• Add an optional TWITTER_BACKEND=xquik posting backend.
• Keep Tweepy as the default backend.
• Route CLI, web UI, and script posting through one shared helper.
• Document the required XQUIK_API_KEY and XQUIK_ACCOUNT settings.

Validation

• python3 -m py_compile cli.py web.py src/functions.py src/instantly-tweet-from-openai.py src/schedule-daily-post-from-openai.py src/schedule-daily-post-from-file.py src/tweeter-random-from-file.py src/tweeter-from-code.py
• git diff --check
• POST https://xquik.com/api/v1/x/tweets without auth returns 401
• Duplicate search found no open or closed Xquik PRs or issues in this repo.

[kriptoburak]

Closing this because it duplicates the existing active Xquik backend PR #3. Sorry for the noise.