Upgrade license to AGPL-3.0 for ActivityPub interop and JSS org alignment #3

Summary

Upgrade the project license from MIT to AGPL-3.0.

Motivation

• Aligns with the licensing direction of other repos in the JavaScriptSolidServer org
• Enables compatibility with ActivityPub ecosystem libraries, which are predominantly AGPL (Mastodon, Lemmy, PeerTube, etc.)
• AGPL ensures grant-funded and community work remains in the commons

Approach

• Original code by Christoph Braun (uvdsl) retains its MIT license notice
• New and combined work licensed under AGPL-3.0-or-later
• MIT → AGPL is a standard, one-way compatible license upgrade