feat(N): workflow pipeline, 3D viewer, worker management, QC tests

- workflow_builder.py: fix broken stubs, add render_order_line_still_task
  (resolves step_path from DB instead of passing order_line_id as step_path)
- domains/rendering/tasks.py: add render_order_line_still_task,
  export_gltf_for_order_line_task, export_blend_for_order_line_task,
  generate_gltf_geometry_task (trimesh STL→GLB, no Blender needed)
- tasks/step_tasks.py: add generate_gltf_geometry_task for CadFile GLB export
- cad router: POST /{id}/generate-gltf-geometry endpoint (admin/PM)
- worker router: GET /celery-workers + POST /scale (docker compose subprocess)
- Dockerfile: pip install -e "[dev]" to enable pytest
- docker-compose.yml: docker socket + compose file mount on backend
- ThreeDViewer.tsx: mode toggle (geometry/production), wireframe, env presets,
  download buttons (GLB + .blend)
- CadPreview.tsx: load gltf_geometry/gltf_production/blend_production assets
  from MediaAsset table and pass URLs to ThreeDViewer
- ProductDetail.tsx: "View 3D" button → /cad/:id, "Generate GLB" button
- media router/service: cad_file_id filter on GET /api/media
- WorkerManagement.tsx: new page with worker status, queue depth, scale controls
- App.tsx + Layout.tsx: /workers route + sidebar link (admin/PM)
- tests: test_rendering_service.py, test_orders_service.py (backend)
- tests: WorkerActivity.test.tsx, WorkerManagement.test.tsx (frontend)

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

backend/app/api/routers/worker.py

@@ -356,6 +356,104 @@ async def cancel_task(task_id: str, user: User = Depends(require_admin_or_pm)):
     return {"revoked": task_id}
 
 
+# ---------------------------------------------------------------------------
+# Worker management — list workers + scale
+# ---------------------------------------------------------------------------
+
+class ScaleRequest(BaseModel):
+    service: str  # "render-worker" | "worker" | "worker-thumbnail"
+    count: int    # 0–20
+
+
+@router.get("/celery-workers")
+async def get_celery_workers(user: User = Depends(require_admin_or_pm)):
+    """List active Celery workers with their queues and active task counts."""
+    import asyncio
+    from app.tasks.celery_app import celery_app
+
+    def _inspect() -> dict:
+        try:
+            insp = celery_app.control.inspect(timeout=2.0)
+            return {
+                "active_queues": insp.active_queues() or {},
+                "active": insp.active() or {},
+                "stats": insp.stats() or {},
+            }
+        except Exception as exc:
+            return {"error": str(exc)}
+
+    data = await asyncio.to_thread(_inspect)
+    if "error" in data:
+        return {"workers": [], "error": data["error"]}
+
+    workers = []
+    for worker_name, queues in data.get("active_queues", {}).items():
+        queue_names = [q.get("name") for q in (queues or [])]
+        active_tasks = data.get("active", {}).get(worker_name, [])
+        stats = data.get("stats", {}).get(worker_name, {})
+        workers.append({
+            "name": worker_name,
+            "queues": queue_names,
+            "active_task_count": len(active_tasks),
+            "active_tasks": [
+                {"name": t.get("name"), "id": t.get("id")} for t in active_tasks
+            ],
+            "total_tasks_processed": stats.get("total", {}),
+        })
+    return {"workers": workers}
+
+
+@router.post("/scale", status_code=http_status.HTTP_202_ACCEPTED)
+async def scale_workers(
+    body: ScaleRequest,
+    user: User = Depends(require_admin_or_pm),
+):
+    """Scale a Compose service (render-worker, worker, worker-thumbnail) up or down.
+
+    Requires the docker socket and compose file to be accessible inside the container
+    (see docker-compose.yml COMPOSE_PROJECT_DIR env var).
+    """
+    import asyncio
+    import os
+    import subprocess
+    from fastapi import HTTPException
+
+    ALLOWED_SERVICES = {"render-worker", "worker", "worker-thumbnail"}
+    if body.service not in ALLOWED_SERVICES:
+        raise HTTPException(400, detail=f"service must be one of {ALLOWED_SERVICES}")
+    if not (0 <= body.count <= 20):
+        raise HTTPException(400, detail="count must be between 0 and 20")
+
+    compose_dir = os.environ.get("COMPOSE_PROJECT_DIR", "/compose")
+    compose_file = os.path.join(compose_dir, "docker-compose.yml")
+
+    def _scale() -> subprocess.CompletedProcess:
+        return subprocess.run(
+            [
+                "docker", "compose",
+                "-f", compose_file,
+                "up",
+                "--scale", f"{body.service}={body.count}",
+                "--no-recreate",
+                "-d",
+            ],
+            capture_output=True, text=True, timeout=120,
+        )
+
+    try:
+        result = await asyncio.to_thread(_scale)
+    except subprocess.TimeoutExpired:
+        raise HTTPException(504, detail="Scale operation timed out")
+
+    if result.returncode != 0:
+        raise HTTPException(
+            500,
+            detail=f"docker compose scale failed: {result.stderr[-500:]}",
+        )
+
+    return {"service": body.service, "count": body.count, "status": "scaling"}
+
+
 # ---------------------------------------------------------------------------
 # Render health check
 # ---------------------------------------------------------------------------