feat(arch): introduce shared/api module for pure interfaces, enums and exceptions

Context

Currently shared/common acts as both the API boundary (interfaces, enums, exceptions) and the implementation layer. ManisGame separates these cleanly into a shared/api module (zero implementations, no platform imports) and a shared/common module (implementations only).

Goal

Extract the following from shared/common into a new :shared:api Gradle module:

• All domain interfaces (e.g. CupService, MapService, LanguageService)
• All enums used across modules
• All domain-specific exception classes

Acceptance Criteria

• :shared:api module exists in settings.gradle.kts
• shared/api has zero implementation classes
• shared/api has no Minestom or Paper imports
• shared/common depends on :shared:api, not the other way around
• ArchUnit rule shared_common_must_not_use_minestom still passes
• ADR written before any code moves

Start with settings.gradle.kts and inspect shared/common to inventory the interfaces, enums, and domain exceptions named in the issue. Write the ADR before moving code, then verify that shared/api contains no implementations or Minestom/Paper imports, shared/common depends on it, and the shared_common_must_not_use_minestom ArchUnit rule still passes.