fix(restacker): refactor and use ProjectContext permissions checks

[jakeaturner]

No description provided.

[Copilot]

Pull request overview

Refactors project permission and book-attachment checks to use the centralized ProjectContext helper (and its permission checks) across Restacker/BookService/Remixer flows, reducing direct project/user lookups in those endpoints.

Changes:

• Extend ProjectContext to always load LibreTexts book attachment fields and add helpers to validate/compose a project’s attached bookID.
• Update BookService.canAccessPage to use ProjectContext for project membership checks.
• Refactor multiple Restacker endpoints to use ProjectContext + ProjectError-based error handling; minor Remixer cleanup for bookID composition.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
server/api/services/project-context.ts	Adds book-attachment fields to permission shape and introduces helpers for attached book ID + validation + new error code.
server/api/services/book-service.ts	Replaces ad-hoc user lookup + permission check with ProjectContext.canMember in canAccessPage.
server/api/restacker.ts	Refactors Restacker endpoints to ProjectContext permission checks and centralized error handling; adjusts restacker run parameters from attached book.
server/api/remixer.ts	Minor refactor to reuse a bookID variable and clarifying comment about when project-book enforcement applies.

Suppressed comments (2)

server/api/services/project-context.ts:151

• Docstring: the composed book ID is library-coverID (not library-bookID). The current wording is confusing and doesn’t match the actual return value (${libreLibrary}-${libreCoverID}).

     * Returns the book ID attached to the project, or null if no book is attached.
     * @param this The ProjectContext instance.
     * @returns An object containing the library, coverID, and the composed bookID (library-bookID), or null if no book is attached.
     */

server/api/services/project-context.ts:130

• assertBookBelongsToProject currently treats any string containing a hyphen as parseable (because getLibraryAndPageFromBookID only checks split('-').length > 1). That means values like lib-123-extra are considered valid and will pass if the first two segments match the project’s book, even though they aren’t valid lib-coverID identifiers per checkBookIDFormat.

        // Check that book ID can be parsed into a valid subdomain and path
        const [subdomain, path] = getLibraryAndPageFromBookID(bookID);
        if (!subdomain || !path) {
            debug("[ProjectContext.assertBookBelongsToProject] Invalid bookID format:", bookID);
            return false;
        }

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The merge-base changed after approval.