feat(PBR): extract Blender PBR properties and apply in 3D viewer

Extract Base Color, Metallic, Roughness, Transmission, IOR from Blender
asset library materials via catalog_assets.py. Store in catalog JSON and
serve via /api/asset-libraries/pbr-map endpoint. Frontend viewers apply
PBR properties to Three.js MeshStandardMaterial using hex color strings
(avoiding Three.js ColorManagement sRGB/linear issues).

Key fixes:
- RLS bypass for material alias lookup in pbr-map endpoint
- pbrMap empty guard prevents premature grey fallback in viewers
- Cache-Control: no-cache on pbr-map requests to avoid stale data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/assetLibraries.ts

@@ -1,7 +1,41 @@
 import api from './client'
 
+// ---------------------------------------------------------------------------
+// PBR material properties (from Blender Principled BSDF)
+// ---------------------------------------------------------------------------
+
+export interface MaterialPBR {
+  base_color: [number, number, number]  // sRGB 0-1
+  metallic: number                       // 0-1
+  roughness: number                      // 0-1
+  transmission?: number                  // 0-1
+  ior?: number                           // typically 1.45
+}
+
+export type MaterialPBRMap = Record<string, MaterialPBR>
+
+export async function fetchMaterialPBR(): Promise<MaterialPBRMap> {
+  const { data } = await api.get<MaterialPBRMap>('/asset-libraries/pbr-map', {
+    headers: { 'Cache-Control': 'no-cache' },
+  })
+  return data
+}
+
+// ---------------------------------------------------------------------------
+// Asset library catalog
+// ---------------------------------------------------------------------------
+
+export interface AssetLibraryCatalogMaterial {
+  name: string
+  base_color?: number[]
+  metallic?: number
+  roughness?: number
+  transmission?: number
+  ior?: number
+}
+
 export interface AssetLibraryCatalog {
-  materials: string[]
+  materials: Array<string | AssetLibraryCatalogMaterial>
   node_groups: string[]
 }