[Backend] Implement GraphQL API endpoint as complement to REST

[Penielka]

Description

The REST API is comprehensive but GraphQL would allow frontend clients to request exactly the data they need, reducing over-fetching and under-fetching. Implement a GraphQL endpoint at /graphql using Apollo Server or express-graphql, with resolvers for the most-used data types: courses, credentials, user profiles, and enrollments.

Definition of Done

• Apollo Server (or express-graphql) integrated with Express
• GraphQL schema defined for: User, Course, Credential, Enrollment, Achievement, Analytics
• Queries: user(id), courses(filter), credential(hash), enrollment(userId, courseId)
• Mutations: enrollInCourse, issueCredential (wraps existing REST logic)
• GraphQL Playground available at /graphql in development
• Authentication/authorization via existing JWT middleware
• Error handling follows GraphQL error spec
• Rate limiting applied to GraphQL endpoint
• Query complexity analysis to prevent expensive queries

Acceptance Criteria

1. POST /graphql accepts queries and returns properly shaped JSON
2. Playground at /graphql works in dev mode
3. Existing REST endpoints continue to work unchanged
4. Auth required for mutations, public queries allowed for course listing
5. N+1 query problem addressed with DataLoader batching

Files to Modify

• backend/package.json — add apollo-server-express, graphql, dataloader
• New file: backend/src/graphql/schema.ts — GraphQL schema definition
• New file: backend/src/graphql/resolvers.ts — resolvers
• New file: backend/src/graphql/context.ts — auth context
• backend/src/index.ts — mount Apollo middleware
• backend/middleware/rateLimit.ts — add GraphQL rate limit tier

[gloskull]

Hello, I have 4 years of experience delivering solutions for projects like this, covering backend, frontend, and smart contract development. After carefully reviewing your requirements, I have a clear understanding of the task and am confident in my ability to deliver a clean, efficient, and thoroughly tested solution on time. I prioritize code quality, maintainability, and industry best practices throughout the development process.
My PR will be submitted in under 10hours.

[clintjeff2]

Hello, I have 6 years of experience working on tasks like this and handling similar challenges across backend, frontend, and smart-contract development. I have carefully read and fully understood the task requirements, and I am confident I can deliver a clean, efficient, and well-tested solution promptly. I always ensure high-quality implementation, maintainability, and adherence to best practices.

Pull request in 5 hours.