fix(critical): SQLAlchemy mapper crash + material matching for USD renders + kanban drag-to-reject

- beat_tasks.py: import app.models at module level so SQLAlchemy can
  resolve relationship("Template") and relationship("User") when domain
  models are imported in isolation inside task functions. Fixes all
  beat tasks (batch_render_notifications, recover_stuck_cad_files) that
  crashed every 60s with mapper initialization error.

- _blender_materials.py: build_mat_map_lower() now adds a slug-normalized
  key variant (re.sub([^a-z0-9]+, _, kl)) for each mat_map entry. OCC
  part names like 'F-802007_TR4-D1-H122AG' → slug 'f_802007_tr4_d1_h122ag'
  now matches USD-imported Blender objects. Existing prefix fallback
  (key.startswith(part_key)) catches AF-suffix variants.

- Orders.tsx: kanban drag-to-reject implemented. submitted/processing
  cards are draggable (cursor-grab). Rejected column highlights with
  red ring on drag-over. Drop opens reject reason modal via createPortal.
  Confirm calls rejectOrder() mutation + invalidates orders cache.

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

backend/app/tasks/beat_tasks.py

@@ -7,6 +7,11 @@ from datetime import datetime, timedelta
 
 from celery import shared_task
 
+# Eagerly import all models so SQLAlchemy can resolve string-based relationship
+# references (e.g. relationship("Template"), relationship("User")) when domain
+# models are imported individually inside task functions.
+import app.models as _all_models  # noqa: F401
+
 logger = logging.getLogger(__name__)