Refactor OPDS links and update catalog metadata

[mekarpeles]

Switch to pyopds2_openlibrary.STORED_SEARCHES ArchiveLabs/pyopds2_openlibrary#18 which uses pyopds2.Search (ArchiveLabs/pyopds2@7b42424)

This PR is not complete.

This pull request refactors and improves the OPDS API endpoints in openlibrary/plugins/openlibrary/api.py. The main changes focus on making the code more modular, reducing duplication, and introducing a new subject-based OPDS endpoint. The update also streamlines how bookshelf and profile links are added, and centralizes the logic for featured subject navigation and stored searches.

Key changes:

Refactoring and Code Simplification:

• Replaces repeated inline bookshelf and profile Link objects with calls to provider.bookshelf_link() and provider.profile_link, making the code more modular and easier to maintain. [1] [2]

New Features:

• Adds a new opds_subjects endpoint (/opds/subjects/<subject_key>) that returns an OPDS catalog for a given subject using search parameters tailored for trending, borrowable books, and safe-for-work content.

Homepage and Navigation Improvements:

• Updates the OPDS homepage to use a more descriptive title ("Browsing the Open Library") and changes subject navigation links to point to the new subject endpoints, improving user navigation and discoverability.
• Refactors the homepage's featured subject groups to use a centralized provider.STORED_SEARCHES list, simplifying the logic for generating trending, classic, romance, kids, thrillers, and textbooks sections.

[mekarpeles]

Code to be moved to https://github.com/archivelabs/opds.openlibrary.org