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/step_tasks.py

@@ -1065,7 +1065,7 @@ def render_order_line_task(self, order_line_id: str):
             except Exception:
                 logger.debug("WebSocket publish skipped (non-fatal)")
 
-            # Notify order creator about render result
+            # Emit per-render activity event (channel=activity, not shown in bell dropdown)
             try:
                 from app.models.order import Order as OrderModel
                 order_row = session.execute(
@@ -1073,7 +1073,7 @@ def render_order_line_task(self, order_line_id: str):
                     .where(OrderModel.id == line.order_id)
                 ).one_or_none()
                 if order_row:
-                    from app.services.notification_service import emit_notification_sync
+                    from app.services.notification_service import emit_notification_sync, CHANNEL_ACTIVITY
                     details: dict = {
                         "order_number": order_row[1],
                         "product_name": product_name,
@@ -1090,9 +1090,10 @@ def render_order_line_task(self, order_line_id: str):
                         entity_type="order",
                         entity_id=str(line.order_id),
                         details=details,
+                        channel=CHANNEL_ACTIVITY,
                     )
             except Exception:
-                logger.exception("Failed to emit render notification")
+                logger.exception("Failed to emit render activity event")
 
             # Check if all lines for this order are done → auto-advance
             order_id_str = str(line.order_id)
@@ -1148,7 +1149,7 @@ def render_order_line_task(self, order_line_id: str):
                         ).one_or_none()
                     eng4.dispose()
                     if order_row2:
-                        from app.services.notification_service import emit_notification_sync
+                        from app.services.notification_service import emit_notification_sync, CHANNEL_ACTIVITY
                         emit_notification_sync(
                             actor_user_id=None,
                             target_user_id=str(order_row2[0]),
@@ -1161,9 +1162,10 @@ def render_order_line_task(self, order_line_id: str):
                                 "output_type": "unknown",
                                 "error": str(exc)[:300],
                             },
+                            channel=CHANNEL_ACTIVITY,
                         )
                 except Exception:
-                    logger.exception("Failed to emit render failure notification")
+                    logger.exception("Failed to emit render failure activity event")
             except Exception:
                 logger.exception(f"Failed to mark {order_line_id} as failed in DB")
             raise