[FEATURE] Replace Postman collection with `.rest` file

[nanotaboada]

Problem

The repository includes a Postman collection for testing the API endpoints. This creates
several friction points:

• Vendor lock-in: Postman is a proprietary tool that requires account sign-in and a
  separate desktop app (or browser extension) to use.
• Redundant with Swagger: The API already exposes interactive documentation at /docs
  (Swagger UI), which covers ad-hoc exploration without any additional tooling.
• Not editor-native: The collection lives outside the development workflow. Developers
  have to context-switch to Postman instead of staying in their editor.
• Format opacity: The .json collection format is not human-readable at a glance and
  is not diff-friendly in code review.

Proposed Solution

Replace postman_collections/ with a rest/players.rest file using the plain-text
HTTP file format,
supported natively by:

• VS Code via the REST Client extension (humao.rest-client)
• IntelliJ IDEA / WebStorm via the built-in HTTP Client

The file is plain text, version-controlled, diff-friendly, and runs directly inside the
editor without leaving the development environment.

Suggested Approach

• Remove postman_collections/
• Add rest/players.rest covering all CRUD operations, with request bodies aligned with
  test fixtures (natural key lookups where needed)
• Add humao.rest-client to .vscode/extensions.json recommendations
• Update Black, Codecov, and flake8 exclusion lists: remove postman_collections/, add
  rest/
• Add an "HTTP Requests" section to README.md
• Update CHANGELOG.md

Acceptance Criteria

• postman_collections/ is removed from the repository
• rest/players.rest covers: health check, POST, GET all, GET by UUID, GET by squad
  number, PUT, DELETE (with two-step UUID resolution for runtime-generated IDs)
• Request bodies and UUIDs in rest/players.rest are aligned with test fixtures
• humao.rest-client is listed in .vscode/extensions.json
• All exclusion lists (Black, Codecov, flake8) updated
• README.md references rest/players.rest and the REST Client extension
• All pre-commit checks pass (flake8, black --check, pytest, coverage ≥ 80%)

References

• REST Client for VS Code
• IntelliJ HTTP Client
• HTTP file format spec

[coderabbitai]

🔗 Related PRs

#326 - feature/aiocache [merged]
#489 - refactor(tests): adopt action-oriented naming convention [merged]

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