FEAT: Adding Scenario run to the REST API

[rlundeen2]

Adds backend endpoints for running, monitoring, and cancelling scenario executions via the REST UI.

• New ScenarioRunService with in-memory run tracking, background asyncio execution, and cancellation support (max 3
  concurrent runs)
• New routes: POST /runs, GET /runs, GET /runs/{id}, DELETE /runs/{id}, GET /runs/{id}/results
• Comprehensive unit tests for service and route layers