Add Xquik export batch API

[kriptoburak]

Summary:

• Add a JSON, JSONL, and CSV Xquik export parser with unit tests.
• Add a POST /analyze-export Flask endpoint that reuses the existing classifier for batch predictions.
• Factor the single-text prediction path and fix the README clone commands.

Duplicate gate:

• Searched the default branch, all PRs, and all issues for Xquik, xquik, x-twitter-scraper, and xquik.com. No existing placement found.

Validation:

• python3 -m unittest test_xquik_export.py
• python3 -m py_compile app.py xquik_export.py test_xquik_export.py
• git diff --check
• staged public diff scan for private terms and restricted outreach text