Replace stale README API docs with published Postman collection #145

Summary

Remove the stale, hand-maintained API reference from README.md and replace it with a properly maintained, published Postman collection + auto-generated docs.

Background

README.md currently contains a large "Detailed API Documentation" section (queries, mutations, request/response examples) that is manually written and drifts out of sync with the actual GraphQL schema. This should be replaced with a Postman collection that is the source of truth, with docs auto-generated from it and published via Postman's built-in Publish feature.

Tasks

• Remove the stale "Detailed API Documentation" section from README.md (Authentication, Tournament Management, Participant Management, Bracket Management, Queries subsections and their request/response examples)
• Build a new Postman collection from scratch covering all current queries and mutations:
  • Auth: registerUser, loginUser, me, refresh token / logout mutations
  • Tournaments: createTournament, updateTournament, deleteTournament, tournaments, tournamentById
  • Participants: addParticipant, joinTournament
  • Brackets/Matches: generateBracket, play, updateRound, matchesForRound
• Add a collection-level description covering what the API does and how to use the collection
• Document authorization in the collection: JWT bearer auth setup, how to obtain a token via loginUser, and which requests require auth
• Add per-request descriptions, example variables, and example responses (success + typed error cases) for each query/mutation
• Generate API documentation from the collection using Postman's built-in doc templates
• Publish the collection and documentation using Postman's Publish feature
• Update README.md to link to the published Postman documentation in place of the removed section

Acceptance Criteria

• README.md no longer contains inline hand-written API request/response examples
• README.md links to the published Postman docs
• Postman collection includes description, auth setup, and all current queries/mutations with examples
• Postman docs are published and publicly accessible via the generated link