feat(creative): implement task cancellation, engine interrupt, and cloud actions (R07-R09, R14, L07)

[BerryUIKI]

Summary

This PR implements comprehensive task cancellation, local engine interruption (ComfyUI & WebUI), active-task shutdown guards, and multi-action cloud execution (inpainting, upscaling, img2img, txt2img).

Key Changes

• Local Engine Interruption:
  • ComfyUIClient.interrupt: Dispatches POST /interrupt to running ComfyUI instances.
  • WebUIRunner.interrupt: Dispatches POST /sdapi/v1/interrupt to running Stable Diffusion WebUI instances.
• Creative Runner Active Task & Cancellation Management:
  • Added active_tasks and active_cancellations registration to CreativeRunner.
  • Added cancel_task(task_id) which signals engine interruption for local engines and truthfully discloses remote computation limitations for cloud providers (R14).
• Cloud Action Dispatch:
  • Implemented _run_cloud across all 4 image actions: txt2img, img2img, inpaint (OpenAI / Fal.ai), and upscale (Fal.ai).
  • Unsupported combinations explain clearly what is missing (R07).
• API Endpoints:
  • GET /api/v1/tasks/active: Lists all running generation and workflow DAG tasks.
  • POST /api/v1/tasks/{task_id}/cancel: Cancels in-flight creative action.
  • POST /api/v1/workflow/cancel/{run_id}: Cancels in-flight DAG workflow.
  • POST /api/v1/manager/shutdown: Guards both creative runner tasks and workflow tasks against silent termination (L07).
• Test Suite:
  • Added backend/tests/test_cancellation_and_actions.py covering task cancellation, cloud disclosures, shutdown guards, and engine interrupt dispatch.