perf: dual queue, GLB caching, WebP output, persistent BVH

Task 4: Dual render queue
- render-worker: heavy (asset_pipeline, concurrency=1) — HQ 2048x2048, animations
- render-worker-light: light (asset_pipeline_light, concurrency=2) — thumbnails, <=1024
- Thumbnails routed to light queue automatically
- Order line renders routed by resolution at dispatch time

Task 5: GLB caching (skip re-tessellation)
- Before tessellating, check if gltf_geometry MediaAsset exists for the cad_file_id
- If found, copy to expected path — render_blender.py finds it and skips tessellation
- Saves 7-11s per re-render of the same product

Task 6: WebP output format
- New 'webp' option in output_format (OutputType admin)
- Blender renders PNG intermediate, Pillow converts to WebP (quality=90, method=4)
- 50-70% smaller files with no visible quality loss
- Correct MIME type (image/webp) in MediaAsset

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/pipeline/tasks/render_thumbnail.py

@@ -72,7 +72,7 @@ def _pipeline_session(tenant_id: str | None = None):
         engine.dispose()
 
 
-@celery_app.task(bind=True, name="app.tasks.step_tasks.render_step_thumbnail", queue="asset_pipeline")
+@celery_app.task(bind=True, name="app.tasks.step_tasks.render_step_thumbnail", queue="asset_pipeline_light")
 def render_step_thumbnail(self, cad_file_id: str):
     """Render the thumbnail for a freshly-processed STEP file.
 
@@ -188,7 +188,7 @@ def render_step_thumbnail(self, cad_file_id: str):
     pl.step_done("render_step_thumbnail")
 
 
-@celery_app.task(bind=True, name="app.tasks.step_tasks.regenerate_thumbnail", queue="asset_pipeline")
+@celery_app.task(bind=True, name="app.tasks.step_tasks.regenerate_thumbnail", queue="asset_pipeline_light")
 def regenerate_thumbnail(self, cad_file_id: str, part_colors: dict):
     """Regenerate thumbnail with per-part colours."""
     pl = PipelineLogger(task_id=self.request.id)