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/pages/NotificationSettings.tsx

@@ -6,13 +6,14 @@ import {
   updateNotificationConfig,
   resetNotificationConfigs,
   type NotificationConfig,
+  type NotificationFrequency,
 } from '../api/notifications'
 
 const EVENT_LABELS: Record<string, string> = {
   'order.submitted': 'Order submitted',
   'order.completed': 'Order completed',
-  'render.completed': 'Render completed',
-  'render.failed': 'Render failed',
+  'render.completed': 'Render completed (activity)',
+  'render.failed': 'Render failed (activity)',
   'excel.imported': 'Excel imported',
 }
 
@@ -22,6 +23,12 @@ const CHANNELS: Array<{ key: 'in_app' | 'email'; label: string; comingSoon?: boo
   { key: 'email', label: 'E-Mail', comingSoon: true },
 ]
 
+const FREQUENCY_OPTIONS: Array<{ value: string; label: string }> = [
+  { value: 'immediate', label: 'Immediate' },
+  { value: 'daily', label: 'Daily summary' },
+  { value: 'never', label: 'Disabled' },
+]
+
 function Toggle({
   enabled,
   disabled,
@@ -61,8 +68,8 @@ export default function NotificationSettingsPage() {
   })
 
   const updateMutation = useMutation({
-    mutationFn: ({ eventType, channel, enabled }: { eventType: string; channel: string; enabled: boolean }) =>
-      updateNotificationConfig(eventType, channel, enabled),
+    mutationFn: ({ eventType, channel, enabled, frequency }: { eventType: string; channel: string; enabled: boolean; frequency?: NotificationFrequency }) =>
+      updateNotificationConfig(eventType, channel, enabled, frequency),
     onSuccess: () => qc.invalidateQueries({ queryKey: ['notification-configs'] }),
     onError: () => toast.error('Failed to update setting'),
   })
@@ -76,15 +83,20 @@ export default function NotificationSettingsPage() {
     onError: () => toast.error('Failed to reset settings'),
   })
 
-  // Build lookup map: eventType+channel → enabled
+  // Build lookup maps
   const configMap = new Map<string, boolean>()
+  const frequencyMap = new Map<string, NotificationFrequency>()
   for (const c of configs) {
     configMap.set(`${c.event_type}:${c.channel}`, c.enabled)
+    frequencyMap.set(`${c.event_type}:${c.channel}`, c.frequency ?? 'immediate')
   }
 
   const isEnabled = (event: string, channel: string) =>
     configMap.get(`${event}:${channel}`) ?? (channel === 'in_app')
 
+  const getFrequency = (event: string, channel: string): NotificationFrequency =>
+    frequencyMap.get(`${event}:${channel}`) ?? 'immediate'
+
   return (
     <div className="p-6 space-y-5 max-w-2xl">
       {/* Header */}
@@ -127,12 +139,15 @@ export default function NotificationSettingsPage() {
                   )}
                 </th>
               ))}
+              <th className="px-4 py-3 text-center text-xs font-semibold text-content-muted uppercase tracking-wide w-40">
+                Frequency
+              </th>
             </tr>
           </thead>
           <tbody className="divide-y divide-border-default">
             {isLoading ? (
               <tr>
-                <td colSpan={CHANNELS.length + 1} className="px-4 py-8 text-center text-sm text-content-muted">
+                <td colSpan={CHANNELS.length + 2} className="px-4 py-8 text-center text-sm text-content-muted">
                   Loading...
                 </td>
               </tr>
@@ -154,6 +169,25 @@ export default function NotificationSettingsPage() {
                       />
                     </td>
                   ))}
+                  <td className="px-4 py-3 text-center">
+                    <select
+                      value={getFrequency(event, 'in_app')}
+                      disabled={updateMutation.isPending || !isEnabled(event, 'in_app')}
+                      onChange={e =>
+                        updateMutation.mutate({
+                          eventType: event,
+                          channel: 'in_app',
+                          enabled: isEnabled(event, 'in_app'),
+                          frequency: e.target.value as NotificationFrequency,
+                        })
+                      }
+                      className="text-xs border border-border-default rounded px-2 py-1 bg-surface text-content disabled:opacity-40"
+                    >
+                      {FREQUENCY_OPTIONS.map(opt => (
+                        <option key={opt.value} value={opt.value}>{opt.label}</option>
+                      ))}
+                    </select>
+                  </td>
                 </tr>
               ))
             )}