feat: Add Custom Population Profile Creator & Editor to Console

- Create `custom_populations` schema inside db_auth.py.
- Implement database helper functions for custom population profiles.
- Add structured modifiers compilation logic supporting *, /, +, -, and Override.
- Hardcode Streamlit app to operate in Calculator mode only, removing JSON input mode.
- Add a first-class "Custom Population Profiles" tab with Creator/Editor and Preview/Quick-Test panels.
- Integrate custom populations into sidebar context selector and exported results.
- Add full unit/integration test suite coverage.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: dfeen87