Rails API implementation for internship task

[roman-mikhaylov]

This pull request implements a Rails API according to the provided OpenAPI schema for managing schools, classes, and students.

Main changes:

• Initialize a Rails 7 API application with PostgreSQL and Docker.
• Implement the following endpoints:
  • GET /schools/{school_id}/classes – returns a list of classes in a school with students_count, returns 404 if the school is not found.
  • GET /schools/{school_id}/classes/{class_id}/students – returns a list of students in a specific class, with 404 responses for missing school or class.
  • POST /students – creates a student, returns the student JSON and an X-Auth-Token header for further deletion.
  • DELETE /students/{id} – deletes a student by ID using an Authorization: Bearer <token> header, with proper error responses for bad ID and missing/invalid token.
• Add models and associations for School, SchoolClass, and Student with basic validations.
• Add simple JSON rendering for successful and error responses.

How to run:

• Start PostgreSQL in Docker (see README for example command).
• Configure config/database.yml for the development environment.
• Run bin/rails db:migrate.
• Run the server with bin/rails s (or docker compose up if configured).
• The API is available at http://127.0.0.1:3000.