- VERSION bumped to 0.8.7
- README.md: comprehensive update with Gokapi integration, stems radio
buttons, download button, cost tracking improvements, stepper fixes,
error recovery, new env vars, new troubleshooting entries
- app.py: updated module docstring with all current routes
- helpers.py: updated module docstring with function group overview,
added section header for Gokapi integration
- config.py: added Gokapi env vars to docstring
- models.py: updated schema comments for per-version costs and
stems_link/stems_url distinction
