Add optional Xquik text posting

[kriptoburak]

Summary

• Add optional Xquik posting via XQUIK_API_KEY and XQUIK_ACCOUNT.
• Keep the existing Twitter, Bluesky, and Mastodon paths unchanged.
• Skip Xquik for local-image posts because this path only passes text.

Notes

The Xquik path calls POST /api/v1/x/tweets with account and text. It is only enabled when both Xquik environment variables are set.

Validation

• python3 -m py_compile papersbot.py
• git diff --check
• Added-line public diff scan
• Local source-truth check for POST /api/v1/x/tweets
• Link check: https://xquik.com/api/v1 returned 200
• Protected route check: Xquik create-tweet returned 401 without credentials