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/services/notification_service.py

@@ -1,3 +1,17 @@
 # Compat shim — use app.domains.notifications.service instead
-from app.domains.notifications.service import emit_notification, emit_notification_sync
-__all__ = ["emit_notification", "emit_notification_sync"]
+from app.domains.notifications.service import (
+    emit_notification,
+    emit_notification_sync,
+    emit_batch_render_notification_sync,
+    CHANNEL_ACTIVITY,
+    CHANNEL_NOTIFICATION,
+    CHANNEL_ALERT,
+)
+__all__ = [
+    "emit_notification",
+    "emit_notification_sync",
+    "emit_batch_render_notification_sync",
+    "CHANNEL_ACTIVITY",
+    "CHANNEL_NOTIFICATION",
+    "CHANNEL_ALERT",
+]

backend/app/services/step_processor.py

@@ -16,40 +16,29 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
-MATERIAL_PALETTE = [
-    "#4C9BE8", "#E85B4C", "#4CBE72", "#E8A84C", "#A04CE8",
-    "#4CD4E8", "#E84CA8", "#7EC850", "#E86B30", "#5088C8",
-]
-
-
-def _material_to_color(material_name: str | None, index: int) -> str:
-    """Return a deterministic hex color: hash material name, or use palette by index."""
-    if material_name and material_name.strip():
-        i = abs(hash(material_name.strip().lower())) % len(MATERIAL_PALETTE)
-        return MATERIAL_PALETTE[i]
-    return MATERIAL_PALETTE[index % len(MATERIAL_PALETTE)]
-
 
 def build_part_colors(
     cad_parsed_objects: list[str],
     cad_part_materials: list[dict],
 ) -> dict[str, str]:
     """
-    Build {part_name: hex_color} for thumbnail rendering.
+    Build {part_name: material_name} for Blender rendering.
+
+    Returns a mapping of part name → Schaeffler material name (e.g. SCHAEFFLER_010101_Steel-Bare).
+    Parts with no material assignment are omitted; Blender will use the fallback material
+    (SCHAEFFLER_059999_FailedMaterial) for unrecognised parts.
 
     Args:
         cad_parsed_objects:  List of part names from cad_file.parsed_objects["objects"].
         cad_part_materials:  List of {part_name, material} dicts from order_item.cad_part_materials.
     """
-    mat_map = {
-        m["part_name"].lower(): m.get("material")
-        for m in cad_part_materials
-        if m.get("part_name")
-    }
-    return {
-        name: _material_to_color(mat_map.get(name.lower()), i)
-        for i, name in enumerate(cad_parsed_objects)
-    }
+    result = {}
+    for m in cad_part_materials:
+        part = m.get("part_name", "").strip()
+        material = m.get("material", "").strip()
+        if part and material:
+            result[part] = material
+    return result
 
 
 def _normalize_stem(name: str) -> str: