refactor(A1): remove Flamenco, simplify render pipeline to Celery-only

- Remove flamenco-manager and flamenco-worker from docker-compose.yml
- Delete flamenco_client.py, flamenco_tasks.py, docker_scaler.py
- Simplify render_dispatcher.py to Celery-only (removes ~300 lines)
- Remove Flamenco beat schedule from celery_app.py
- Clean admin.py: remove flamenco settings, endpoints, threejs validation
- Clean orders.py cancel-render: Celery revoke only
- Clean worker.py: remove flamenco_job_id from activity response
- Migration 032: cancel lingering flamenco jobs, remove flamenco settings
- PLAN.md: mark all decisions confirmed, status IN UMSETZUNG

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routers/worker.py

@@ -38,7 +38,6 @@ class RenderJobEntry(BaseModel):
     output_type_name: str | None
     render_status: str
     render_backend_used: str | None
-    flamenco_job_id: str | None
     render_started_at: str | None
     render_completed_at: str | None
     updated_at: str
@@ -140,7 +139,6 @@ async def get_worker_activity(
             output_type_name=rl.output_type.name if rl.output_type else None,
             render_status=rl.render_status,
             render_backend_used=rl.render_backend_used,
-            flamenco_job_id=rl.flamenco_job_id,
             render_started_at=rl.render_started_at.isoformat() if rl.render_started_at else None,
             render_completed_at=rl.render_completed_at.isoformat() if rl.render_completed_at else None,
             updated_at=rl.updated_at.isoformat(),