Add CHANGELOG.md + migrate PHPUnit annotations to attributes

[Chemaclass]

Context

Two small DX/housekeeping items now that the tooling is on PHPUnit 10 / PHPStan 2 / Rector 2:

• No CHANGELOG.md exists; the recent DX/upgrade work is only visible in git history.
• Tests still use PHPUnit doc-annotations (@runInSeparateProcess, @preserveGlobalState disabled) — PHPUnit 10 supports PHP 8 attributes (#[RunInSeparateProcess], #[PreserveGlobalState(false)]), which are type-checked and won't silently no-op if mistyped.
  • tests/Unit/Invoice/Infrastructure/Middleware/CorsMiddlewareTest.php
  • tests/Unit/Invoice/Infrastructure/Handler/InvoiceExceptionHandlerTest.php

Goal

Add a changelog and modernize test metadata to attributes.

Scope / Tasks

• Add CHANGELOG.md following Keep a Changelog. Seed it from the merged history: gacela 1.19 / router 0.13 upgrade, LUD-06 pr key fix, CORS middleware + global exception handler, nostr.json→backends.json rename, BackendType/ConfigKey/addBackend, PHPUnit 10 / PHPStan 2 / Rector 2. Note the breaking changes (bolt11→pr, file rename).
• Migrate the two test classes' @runInSeparateProcess / @preserveGlobalState disabled doc-annotations to #[RunInSeparateProcess] / #[PreserveGlobalState(false)] attributes.
• (Optional) sweep for any other PHPUnit doc-annotations and convert to attributes for consistency.

Acceptance criteria

• CHANGELOG.md present and accurate.
• The two process-isolated tests use attributes and still pass.
• composer test-all green.

References

• Keep a Changelog: https://keepachangelog.com/
• PHPUnit attributes: https://docs.phpunit.de/en/10.5/attributes.html