🐛 Fix import cycles

[flovouin]

📝 Description of the PR

Removes the circular imports present in the nestjs sources. Three cycles existed: between the error DTOs and the OpenAPI decorators, within the pagination decorators, and between the healthcheck barrel and its module.

Because the project compiles with verbatimModuleSyntax, an import { X } statement is emitted verbatim into the JavaScript output even when X is only ever used in a type position. Several OpenAPI and pagination decorators imported classes purely for typing, which turned type-only dependencies into real runtime edges. These are now import type statements, which are fully erased at compile time. Modules that reached for a sibling barrel re-exporting them now import the specific file they need instead.

The error DTO cycle was the one that could actually fail at runtime: try-map.ts builds its error cases at module load time, so entering the cycle from the wrong side left a DTO class in its temporal dead zone. The healthcheck cycle was latent rather than live, as the shared constant is only read inside a static factory method; it would have become a genuine failure as soon as anything used it during module evaluation. To break it, HEALTHCHECK_ENDPOINT moved out of the healthcheck barrel into its own module, and the barrel re-exports it.

The public API is unchanged: every symbol is still exported from the same package entry points as before.

📋 Check list

• 🧪 Unit tests have been written.
• 📝 Documentation has been updated.