refactor: improve python code boundaries

[pixelcola]

Summary

• Add a shared CLI error-reporting decorator so Typer command bodies keep the main flow visible
• Split profile download into fetch, validate, and write steps with narrower error boundaries
• Document the CLI error boundary convention in backend specs

Tests

• UV_CACHE_DIR=/tmp/uv-cache uv run pytest
• UV_CACHE_DIR=/tmp/uv-cache uv run ruff check src tests
• UV_CACHE_DIR=/tmp/uv-cache uv run ty check src tests