Add godoc comments to the exported auth package helpers. Expand the README
with feature and tech stack overviews, setup instructions, an API reference,
and make targets for development workflows.
Move HTTP handler logic out of main into internal api and server packages,
slimming main to a thin entrypoint. Bundles bug fixes: login nil-panic,
empty-password validation, chirp sort ordering, duplicate-email 409, and
restricting the file server to the assets directory.