feat(phase5.1+6): fallback material cleanup + notification batch refactor

Phase 5.1 — MATERIAL_PALETTE removal:
- Remove MATERIAL_PALETTE + _material_to_color() from step_processor.py
- build_part_colors() now returns {part→material_name} for Blender resolver

Phase 6 — Notification Center Refactor:
- Migration 051: add channel (activity|notification|alert) to audit_log,
  add frequency (immediate|daily|never) to notification_configs
- Three notification channels: activity (per-render), notification (batch
  order summaries), alert (admin infrastructure)
- Per-render emit_notification_sync calls demoted to channel=activity
- New emit_batch_render_notification_sync(): single summary notification
  when all order lines reach terminal state ("47/50 succeeded, 3 failed")
- Beat task batch_render_notifications every 60s: safety-net for missed
  batch notifications after order completion
- GET /notifications: defaults to channel IN (notification, alert);
  accepts ?channel=activity for activity feed
- Unread count badge counts only notification+alert channels
- Notifications.tsx: three tabs (Notifications | Activity | Alerts)
- NotificationSettings.tsx: frequency dropdown per event type

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

backend/app/domains/orders/service.py

@@ -81,19 +81,12 @@ def check_order_completion(order_id: str) -> bool:
             session.commit()
             logger.info(f"Order {order_id} auto-advanced to completed (all {len(lines)} lines done)")
 
-            # Notify order creator
+            # Emit a single batch notification summarising all render results
             try:
-                from app.domains.notifications.service import emit_notification_sync
-                emit_notification_sync(
-                    actor_user_id=None,
-                    target_user_id=str(order.created_by),
-                    action="order.completed",
-                    entity_type="order",
-                    entity_id=str(order_id),
-                    details={"order_number": order.order_number},
-                )
+                from app.domains.notifications.service import emit_batch_render_notification_sync
+                emit_batch_render_notification_sync(order_id)
             except Exception:
-                logger.exception("Failed to emit order.completed notification")
+                logger.exception("Failed to emit batch render notification for order %s", order_id)
 
             return True