feat: add GraphQL gateway with cursor pagination

[playmaker410]

Description

Adds a typed GraphQL gateway at POST /api/v1/graphql over the existing NestJS services.

The gateway provides:

• approved agent reviews with opaque, versioned cursor pagination
• stable createdAt DESC and id DESC keyset ordering
• request-local DataLoader batching for review authors
• typed rating aggregation queries
• generated TypeScript operation/result types and TypedDocumentNode values
• reusable typed frontend client helpers and pagination example
• production introspection protection and authenticated HTTP contract coverage
• CI checks for generated SDK freshness, gateway tests, pagination tests, type checking, and builds

Business logic remains in the existing AgentReviewsService and UserService instead of being duplicated in GraphQL resolvers.

Closes #130

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Performance improvement
• Code refactoring

How Has This Been Tested?

• Unit tests added/updated
• E2E/HTTP contract tests added/updated
• Manual generation and build verification performed
• Tested in staging environment

Commands run:

• npm run graphql:codegen
• npm test -- --runInBand src/graphql src/discovery/reviews/agent-reviews.service.spec.ts src/common/pagination/cursor-pagination.service.spec.ts (39 tests passed)
• npm run build
• ESLint on the changed GraphQL TypeScript files

The complete repository suite reports 816 passing tests and 2 unrelated pre-existing failures in challenge.service.spec.ts and websocket.stress.spec.ts. The scoped GraphQL and pagination suites are green and are now enforced in CI.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented hard-to-understand areas
• I have made corresponding documentation changes
• My changes generate no new warnings
• I have added tests that prove the feature works
• New feature tests pass locally
• Any dependent changes have been merged and published in downstream modules (not applicable)

Screenshots

Not applicable; this contribution adds an API and generated TypeScript SDK surface.

Additional Notes

The schema is packaged as a Nest build asset and can be loaded independently of the process working directory. Frontends can import generated documents, operation types, and helpers from src/graphql/client/index.ts.