feat: BLENDER_CINEMATIC workflow graph node (M1)

Add StepName.BLENDER_CINEMATIC and full graph runtime support so cinematic
output types can be promoted from legacy_only to graph/shadow rollout mode.

- process_steps.py: add BLENDER_CINEMATIC = "blender_cinematic" enum value
- workflow_executor.py: map to render_cinematic_task in STEP_TASK_MAP
- workflow_node_registry.py: node definition with render/scene/camera fields
  (no animation params — cinematic is fixed at 250 frames @ 25fps)
- workflow_graph_runtime.py: _ORDER_LINE_RENDER_STEPS, _CINEMATIC_TASK_KEYS,
  shadow queue routing, predict_render_output_artifact (mp4),
  _build_task_kwargs, _artifact_kind_override_for_step
- tasks.py: _normalize_cinematic_params + render_cinematic_task Celery task
  (calls render_cinematic_to_file, publishes as turntable asset type since mp4)

No DB migration needed: admins can now manually set cinematic output types
to graph rollout mode via the admin panel and assign a workflow definition.

docs: learnings erfasst — BLENDER_CINEMATIC workflow graph node M1

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

backend/app/core/process_steps.py

@@ -25,6 +25,7 @@ class StepName(StrEnum):
     RESOLVE_TEMPLATE = "resolve_template"
     BLENDER_STILL = "blender_still"
     BLENDER_TURNTABLE = "blender_turntable"
+    BLENDER_CINEMATIC = "blender_cinematic"
     OUTPUT_SAVE = "output_save"
 
     # ── Asset export ──────────────────────────────────────────────────