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/alembic/versions/051_notification_channels.py

@@ -0,0 +1,43 @@
+"""Add channel to audit_log and frequency to notification_configs.
+
+Revision ID: 051
+Revises: 050
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "051"
+down_revision = "050"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Add channel column to audit_log
+    op.add_column(
+        "audit_log",
+        sa.Column(
+            "channel",
+            sa.String(20),
+            nullable=False,
+            server_default="notification",
+        ),
+    )
+    op.create_index("ix_audit_log_channel", "audit_log", ["channel"])
+
+    # Add frequency column to notification_configs
+    op.add_column(
+        "notification_configs",
+        sa.Column(
+            "frequency",
+            sa.String(20),
+            nullable=False,
+            server_default="immediate",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("notification_configs", "frequency")
+    op.drop_index("ix_audit_log_channel", table_name="audit_log")
+    op.drop_column("audit_log", "channel")