feat(faustwp): versioned document store with conflict detection #2562

Teams that need to persist app data in WordPress (settings, drafts, structured documents) each end up writing the same small plugin: a custom post type, a REST route, and some kind of version check so concurrent writers don't overwrite each other.

Proposal

Ship a supported, versioned document store in FaustWP:

• A custom post type holding JSON documents keyed by slug.
• REST routes under faustwp/v1 to read and write documents.
• Every write includes the version it was based on. If the stored version has moved on, the route returns 409 Conflict with the current version so the client can merge or retry.
• Permissions go through standard capabilities, so it composes with the scoped service user proposed in feat(faustwp): scoped service user for server-side writes #2561.

Acceptance

• GET returns the document and its version.
• A PUT with a matching version succeeds and bumps the version.
• A PUT with an old version returns 409 and does not modify the document.
• Covered by PHPUnit tests and documented.