Fix upload proxy auth fallback

[ayushshrivastv]

Summary

• add a shared internal auth helper for chat-app to data-manager requests
• fall back to a deterministic token derived from PG_PASSWORD when DM_API_TOKEN is not configured
• keep explicit DM_API_TOKEN precedence and add regression coverage for the auth path

Why

Issue #517 reports upload actions failing with Data manager authentication failed while the data-manager logs show a 302 on /document_index/upload_url. This happens when data-manager auth is enabled but the internal proxy token is not configured, so the chat app gets redirected to the uploader login page instead of being treated as a trusted internal caller.

Testing

• .venv/bin/python -m pytest tests/unit/test_internal_auth.py -q
• python3 -m compileall src/utils/internal_auth.py src/interfaces/uploader_app/app.py src/interfaces/chat_app/app.py tests/unit/test_internal_auth.py

[ayushshrivastv]

Verified locally on macOS in Docker with data_manager.auth.enabled: true and no DM_API_TOKEN set. Both URL upload/scrape and PDF upload from the Upload Data UI now succeed instead of failing with the previous data-manager auth redirect/error.