ci: migrate coverage to Codecov and reuse workflows

[ohmyfelix]

What changed

• switched the README coverage badge from Coveralls to Codecov and removed the obsolete tests/.coveralls.yml
• replaced the repo-specific coverage upload job with the shared nette-tester-coverage-v2 workflow and wired it to make coverage-clover
• split the old monolithic CI into reusable contributte workflow callers for CodeSniffer, PHPStan, and Nette Tester
• dropped the legacy custom matrix in favor of the shared contributte-style PHP 8.2-8.4 plus lowest-deps test lanes

Why

• finishes the Coveralls-to-Codecov migration for this repository
• aligns contributte/thepay-api CI with the standard contributte workflow layout and shared maintenance model

Refs contributte/contributte#72

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️