chore: justfile + license maintenance

[adamXbot]

• Adds a justfile delegating to the repo's npm scripts: lint runs Mintlify validate + broken-links (as CI does), run serves the docs locally. (This repo has no check/sync-changelog scripts — its package.json exposes validate, check-links, and dev, so the justfile delegates to those.)
• Adds Apache-2.0 LICENSE (holder: PrivacyKey, 2026), matching its sibling docs repos; siblings carry no NOTICE, so none added here.

Verification (just --list)

Available recipes:
    default # List available commands

    [dev]
    lint    # Validate the Mintlify build and check for broken links (as CI does)
    run     # Serve the docs locally with Mintlify

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