Add Kotlin vs Scala API migration spike findings to README

[Copilot]

This spike captures practical options for introducing Kotlin or Scala into the API codebase and makes the trade-offs explicit for near-term planning. It documents partial and broader adoption paths and provides a default direction for future implementation work.

• What this PR adds

  • Introduces a new README.md section: “Spike: Kotlin / Scala options for API source”.
  • Adds a compact comparison of three paths:
    • Java-first with incremental Kotlin in selected API packages
    • Broad Kotlin migration over time
    • Scala migration for some/all API code

• Decision framing included

  • Records pros/risks per option (interop, migration effort, ecosystem fit, onboarding cost).
  • States a working recommendation: Kotlin-first incremental adoption, while keeping existing Java modules stable unless touched by feature work.

• Documentation-only scope

  • No production code, dependencies, or build configuration changes.

## Spike: Kotlin / Scala options for API source

| Option | Scope | Pros | Risks / Cost |
| --- | --- | --- | --- |
| Keep Java as primary, add Kotlin for selected API packages first | Some API source code | Lowest migration risk, full Spring + JVM interop | Mixed-language standards to maintain |
| Move most API code to Kotlin over time | Most API source code | Null-safety, concise syntax, strong Spring ecosystem support | Medium migration/testing effort |
| Move API to Scala | Some or all API source code | Powerful FP model and type system | Highest migration and onboarding cost |

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