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>

frontend/src/api/notifications.ts

@@ -1,5 +1,7 @@
 import api from './client'
 
+export type NotificationChannel = 'notification' | 'activity' | 'alert'
+
 export interface Notification {
   id: string
   action: string
@@ -8,6 +10,7 @@ export interface Notification {
   details: Record<string, unknown> | null
   timestamp: string
   read_at: string | null
+  channel?: NotificationChannel
 }
 
 export interface NotificationListResponse {
@@ -20,6 +23,7 @@ export async function getNotifications(params?: {
   limit?: number
   offset?: number
   unread_only?: boolean
+  channel?: NotificationChannel
 }): Promise<NotificationListResponse> {
   const { data } = await api.get('/notifications', { params })
   return data
@@ -40,12 +44,15 @@ export async function markOneAsRead(id: string): Promise<void> {
 
 // ── Notification Config ───────────────────────────────────────────────────
 
+export type NotificationFrequency = 'immediate' | 'daily' | 'never'
+
 export interface NotificationConfig {
   id: string
   user_id: string
   event_type: string
   channel: 'in_app' | 'email'
   enabled: boolean
+  frequency: NotificationFrequency
   created_at: string
 }
 
@@ -57,11 +64,14 @@ export async function getNotificationConfigs(): Promise<NotificationConfig[]> {
 export async function updateNotificationConfig(
   eventType: string,
   channel: string,
-  enabled: boolean
+  enabled: boolean,
+  frequency?: NotificationFrequency
 ): Promise<NotificationConfig> {
+  const body: Record<string, unknown> = { enabled }
+  if (frequency !== undefined) body.frequency = frequency
   const res = await api.put<NotificationConfig>(
     `/notifications/config/${encodeURIComponent(eventType)}/${channel}`,
-    { enabled }
+    body
   )
   return res.data
 }