PyPI downloads example: total chart, direct line, in-memory cache

[jlowin]

The PyPI downloads example previously required pre-fetching data to CSV before the server could start, and only showed dependent package downloads. This rework makes the example self-contained and more informative.

The dashboard now shows the package's own total weekly downloads in a chart above the dependents chart. A "direct" line in the dependents chart estimates downloads not attributable to any tracked dependent (total minus sum of top-20 dependents). A checkbox toggles the direct line instantly via a lightweight /api/render endpoint that re-renders from an in-memory cache without re-querying ClickHouse.

The page header is reactive — it updates as you type in the package name field, using the defer/insert pattern:

pkg_input = Input(name="package_name", value="fastmcp", defer=True)

with Column():
    Text(f"PyPI Downloads: {pkg_input.rx}")  # reactive header
    with Form(...):
        insert(pkg_input)  # render the input here

All file-based caching (CSV, meta JSON) is replaced with a simple in-memory dict, so the server starts clean and fetches data on demand.