refactor(section-a+c): decompose step_tasks.py into pipeline domain

Section A — 1172-line step_tasks.py split into 4 focused modules under
backend/app/domains/pipeline/tasks/:
- extract_metadata.py: process_step_file, reextract_cad_metadata,
  _auto_populate_materials_for_cad, bbox helpers + PipelineLogger
- render_thumbnail.py: render_step_thumbnail, regenerate_thumbnail
  + PipelineLogger instrumentation
- render_order_line.py: dispatch_order_line_render, render_order_line_task
  (full still+turntable paths) + PipelineLogger step tracking
- export_glb.py: generate_gltf_geometry_task, generate_gltf_production_task

All task names preserved (app.tasks.step_tasks.*) for backward compat.
step_tasks.py is now a 23-line import-only shim.

Section C — celery_app.py task routing:
- app.domains.pipeline.tasks.* → step_processing (primary route)
- All 4 new modules added to include list for Celery discovery
- app.tasks.step_tasks.* kept as legacy fallback for in-flight tasks

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

backend/app/tasks/celery_app.py

@@ -7,7 +7,11 @@ celery_app = Celery(
     broker=settings.redis_url,
     backend=settings.redis_url,
     include=[
-        "app.tasks.step_tasks",
+        "app.tasks.step_tasks",  # shim — re-exports from domains.pipeline.tasks.*
+        "app.domains.pipeline.tasks.extract_metadata",
+        "app.domains.pipeline.tasks.render_thumbnail",
+        "app.domains.pipeline.tasks.render_order_line",
+        "app.domains.pipeline.tasks.export_glb",
         "app.tasks.ai_tasks",
         "app.tasks.beat_tasks",
         "app.domains.rendering.tasks",
@@ -24,10 +28,12 @@ celery_app.conf.update(
     timezone="UTC",
     enable_utc=True,
     task_routes={
-        "app.tasks.step_tasks.*": {"queue": "step_processing"},
-        "app.tasks.ai_tasks.*": {"queue": "ai_validation"},
-        "app.tasks.beat_tasks.*": {"queue": "step_processing"},
+        "app.domains.pipeline.tasks.*": {"queue": "step_processing"},
         "app.domains.rendering.tasks.*": {"queue": "thumbnail_rendering"},
+        "app.tasks.beat_tasks.*": {"queue": "step_processing"},
+        "app.tasks.ai_tasks.*": {"queue": "ai_validation"},
+        # Legacy task names (shim) — keep until old queued tasks drain
+        "app.tasks.step_tasks.*": {"queue": "step_processing"},
     },
     beat_schedule={
         "broadcast-queue-status-every-10s": {