feat: add standalone Plot System integration API

[Zoriot]

Summary

Add a standalone plot-system-api module for Plot System integrations.

The plugin now exposes stable lifecycle events with immutable API DTOs, allowing integrations such as DiscordPlotSystem to depend on a small, independently publishable API instead of the full Plot System implementation.

Changes

• Add the plot-system-api Gradle module.
• Add stable lifecycle events for:
  • Plot creation
  • Plot submission and undo submission
  • Plot review and rejection
  • Plot abandonment
  • Inactivity warnings
  • Feedback updates
  • Review undo
• Add immutable API models:
  • PlotSnapshot
  • ReviewSnapshot
  • PlotStatus
  • PlotType
  • PlotDifficulty
  • PlotAbandonReason
• Add API-to-domain snapshot conversion inside the plugin module.
• Configure the API module for independent Maven publication.
• Keep integration code independent from Plot System implementation classes.

Motivation

External integrations previously had to depend on internal classes such as Plot and PlotReview. That coupled integrations to implementation details and prevented independent API versioning.

Integrations can now depend only on:

com.alpsbte:plot-system-api