Add Xquik CSV batch input support

[kriptoburak]

Summary

• Add a backend text-source helper that preserves the existing texts array contract and also extracts text from tabular record payloads.
• Support Xquik export rows via Tweet Text while filtering blank values before sentiment analysis.
• Return the detected source column in the sentiment API response and add focused unit tests for direct arrays, Xquik records, and missing text fields.

Duplicate Checks

• Repository tree search found no existing Xquik, Tweet Text, or Tweet Created At support.
• REST PR and issue checks returned no matching Xquik or same-surface entries.

Validation

• PYTHONPATH=backend python3 -m unittest backend/tests/test_text_sources.py
• python3 -m py_compile backend/services/text_sources.py backend/app.py backend/tests/test_text_sources.py
• git diff --check
• Public diff safety scan for private details, unsafe badge domains, tokens, costs, and unsupported claims