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/notifications/schemas.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 import uuid
 from datetime import datetime
+from typing import Optional
 from pydantic import BaseModel
 
 
@@ -11,6 +12,7 @@ class NotificationConfigOut(BaseModel):
     event_type: str
     channel: str
     enabled: bool
+    frequency: str = "immediate"
     created_at: datetime
 
     model_config = {"from_attributes": True}
@@ -18,3 +20,4 @@ class NotificationConfigOut(BaseModel):
 
 class NotificationConfigUpdate(BaseModel):
     enabled: bool
+    frequency: Optional[str] = None  # "immediate" | "daily" | "never"