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/tasks/beat_tasks.py

@@ -80,3 +80,74 @@ def recover_stuck_cad_files() -> None:
             logger.debug("recover_stuck_cad_files: no stuck files found")
     except Exception as exc:
         logger.error("recover_stuck_cad_files failed: %s", exc)
+
+
+@shared_task(name="app.tasks.beat_tasks.batch_render_notifications", queue="step_processing")
+def batch_render_notifications() -> None:
+    """Check for orders where all lines are terminal but no batch notification emitted yet.
+
+    This acts as a safety net for batch notifications that may have been missed
+    if the order completion hook failed or was skipped. Runs every 60 seconds.
+    """
+    try:
+        from sqlalchemy import create_engine, select
+        from sqlalchemy.orm import Session
+        from app.config import settings
+        from app.domains.orders.models import Order, OrderStatus
+        from app.domains.notifications.models import AuditLog
+        from app.domains.notifications.service import (
+            emit_batch_render_notification_sync,
+            CHANNEL_NOTIFICATION,
+        )
+
+        sync_url = settings.database_url.replace("+asyncpg", "")
+        engine = create_engine(sync_url)
+
+        with Session(engine) as session:
+            # Find all completed orders
+            completed_order_ids: list[str] = [
+                str(oid) for oid in session.execute(
+                    select(Order.id).where(Order.status == OrderStatus.completed)
+                ).scalars().all()
+            ]
+
+            if not completed_order_ids:
+                engine.dispose()
+                logger.debug("batch_render_notifications: no completed orders found")
+                return
+
+            # Find which of those already have a batch notification
+            notified_ids_raw = session.execute(
+                select(AuditLog.entity_id).where(
+                    AuditLog.entity_type == "order",
+                    AuditLog.action == "order.completed",
+                    AuditLog.channel == CHANNEL_NOTIFICATION,
+                    AuditLog.entity_id.in_(completed_order_ids),
+                )
+            ).scalars().all()
+            notified_ids = set(notified_ids_raw)
+
+        engine.dispose()
+
+        orders_needing_notification = [
+            oid for oid in completed_order_ids if oid not in notified_ids
+        ]
+
+        if not orders_needing_notification:
+            logger.debug("batch_render_notifications: no orders need batch notification")
+            return
+
+        logger.info(
+            "batch_render_notifications: emitting batch notifications for %d order(s)",
+            len(orders_needing_notification),
+        )
+        for order_id in orders_needing_notification:
+            try:
+                emit_batch_render_notification_sync(str(order_id))
+            except Exception as exc:
+                logger.error(
+                    "batch_render_notifications: failed for order %s: %s", order_id, exc
+                )
+
+    except Exception as exc:
+        logger.error("batch_render_notifications failed: %s", exc)