Add optional Xquik post backend

[kriptoburak]

Summary

• add an opt-in POST_BACKEND=xquik path for posts
• send Reddit image URLs to Xquik as public media URLs
• keep Tweepy as the default backend and document the Xquik settings

Validation

• python3 -m py_compile pyReddit2Twitter/app.py
• git diff --check
• checked edited files for em dash and banned double-hyphen spacing
• checked the repo for credential-like literals before opening the PR
• checked open and closed PRs for existing Xquik work
• confirmed POST /api/v1/x/tweets accepts public media URLs