feat: migrate to zeitwerk

[gildesmarais]

This pull request introduces a major refactor of the backend code structure to adopt the Zeitwerk autoloader, improves backend maintainability, and updates development and linting workflows. It removes manual requires and directory-specific namespace wiring, reorganizes files under the app/web/** directory, and ensures that all code is autoloaded in a consistent, Rails-like fashion. Additionally, it adds a Zeitwerk eager-load check to the linting process and updates documentation accordingly. Several files are renamed or removed to reflect the new structure, and some error-handling and validation logic is refactored for clarity and consistency.

Backend autoloading and structure improvements:

• Introduced the zeitwerk gem and removed the rack-unreloader gem, enabling Zeitwerk-based autoloading for backend code under the Html2rss::Web namespace, and updated app.rb to use Html2rss::Web::Boot for setup and eager loading. [1] [2]
• Moved backend files into the app/web/** directory to match Zeitwerk's expectations, and removed manual require statements in favor of autoloading. [1] [2] [3] [4] [5] [6] [7]

Development and linting workflow updates:

• Added a Zeitwerk eager-load verification task (rake zeitwerk:verify) to the Makefile linting process and as part of make ready, ensuring loader drift is caught early in development. [1] [2] [3]

Error handling and validation refactoring:

• Updated error handling in API modules to reference error classes via the fully qualified Html2rss::Web namespace, and refactored validation logic in CreateFeed for clarity and maintainability. [1] [2] [3] [4] [5]

Roda plugin and middleware changes:

• Added the :head and :not_allowed plugins to Roda configuration, improving HTTP method handling.

Documentation updates:

• Updated README.md to explain the new backend file structure, Zeitwerk integration, and contributor guidelines for backend code placement.