[Documentation] Update README to reflect actual project structure

[ConnorQi01]

Summary

Update README to match the actual codebase. Remove references to files and features that do not exist.

Changes

• Remove non-existent files from structure tree (db_util.py, data_loader.py, screenshots/, api_scenarios.py, api_interface.md)
• Replace Jenkins CI/CD references with GitHub Actions
• Remove DB layer from architecture diagram
• Delete Jenkins CI/CD integration section
• Fix run commands (add -n 0 for UI tests, correct API host)

Test Plan

• Verify structure tree matches actual files on disk
• Verify all run commands are accurate

Closes #14