Add hide_url() high-level helper + README quickstart

[joeollis]

Problem

Mirror of layervai/qurl-typescript#54. Python SDK exposes the raw two-step model; an AI asked to "hide a URL" needs a single-call entry point.

Expected

link = client.hide_url(
    target_url="https://internal.example.com",
    expires_in=60,
    custom_domain=...,   # optional
    policy=...,          # optional
)
# link.url, link.expires_at

Internally does the resource+mint_link dance. README quickstart leads with this.

Related

Epic: layervai/qurl-service#370
Paired: layervai/qurl-typescript#54