refactor(A3): remove threejs-renderer service, add migration 033

- Migration 033: reset threejs thumbnail_renderer setting → blender,
  remove obsolete threejs_render_size system_setting
- step_processor.py threejs code paths already fall through to Pillow
  (committed in A2); Three.js browser viewer (ThreeDViewer.tsx) remains
- threejs-renderer/ directory kept for reference but no longer built

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

backend/alembic/versions/033_remove_threejs_renderer.py

@@ -0,0 +1,37 @@
+"""Remove threejs-renderer: reset thumbnail_renderer setting to 'blender'.
+
+CadFiles that were rendered with three.js will be re-rendered on the next
+"Regenerate All Thumbnails" admin action.
+
+Revision ID: 033
+Revises: 032
+Create Date: 2026-03-06
+"""
+from alembic import op
+
+revision = '033'
+down_revision = '032'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Switch any existing 'threejs' renderer setting to 'blender'
+    op.execute("""
+        UPDATE system_settings
+        SET value = 'blender'
+        WHERE key = 'thumbnail_renderer' AND value = 'threejs'
+    """)
+
+    # Remove obsolete threejs_render_size setting (no longer used)
+    op.execute("""
+        DELETE FROM system_settings WHERE key = 'threejs_render_size'
+    """)
+
+
+def downgrade():
+    op.execute("""
+        INSERT INTO system_settings (key, value, updated_at)
+        VALUES ('threejs_render_size', '1024', NOW())
+        ON CONFLICT (key) DO NOTHING
+    """)