Add Xquik export prediction

[kriptoburak]

Summary

• Add a stdlib Xquik export parser for JSON, JSONL, and CSV tweet files.
• Add batch and Xquik-export prediction endpoints behind the existing FastAPI app.
• Validate blank tweet input and fix the documented app entry point and response field.

Validation

• python3 -m pytest -q test_placeholder.py
• python3 -m py_compile xquik_import.py RedBricks.py test_placeholder.py
• git diff check

Duplicate gate

• Checked the default branch for existing Xquik references.
• Checked all-state pull requests and issues through the GitHub API. No Xquik hits were present.