feat: sharp edge pipeline V02, tessellation presets, media cache-bust, GMSH plan

Sharp Edge Pipeline V02:
- export_step_to_gltf.py: replace BRep_Tool.Polygon3D_s (returns None in XCAF) with
  GCPnts_UniformAbscissa curve sampling at 0.3mm step — extracts 17,129 segment pairs
- Inject sharp_edge_pairs + sharp_threshold_deg into GLB extras (scenes[0].extras)
  via binary GLB JSON-chunk patching (no extra dependency)
- export_gltf.py: read schaeffler_sharp_edge_pairs from Blender scene custom props,
  apply via KD-tree to mark edges sharp=True + seam=True (OCC mm Z-up → Blender transform)
- tools/restore_sharp_marks.py: dual-pass (dihedral angle + OCC pairs), updated coordinate
  transform (X, -Z, Y) * 0.001

Tessellation:
- Admin UI: Draft / Standard / Fine preset buttons with active-state highlighting
- Default angular deflection: preview 0.5→0.1 rad, production 0.2→0.05 rad
- export_glb.py: read updated defaults from system_settings

Media / Cache:
- media/service.py: get_download_url appends ?v={file_size_bytes} cache-buster
- media/router.py: Cache-Control: no-cache for all download/thumbnail endpoints

Render pipeline:
- still_render.py / turntable_render.py: shared GPU activation + camera improvements
- render_order_line.py: global render position support
- render_thumbnail.py: updated defaults

Frontend:
- InlineCadViewer: file_size_bytes-aware URL update triggers re-fetch on regeneration
- ThreeDViewer: material panel, part selection, PBR mode improvements
- Admin.tsx: tessellation preset cards, GMSH setting dropdown
- MediaBrowser, ProductDetail, OrderDetail, Orders: various UI improvements
- New: MaterialPanel, GlobalRenderPositionsPanel, StepIndicator components
- New: renderPositions.ts API client

Plans / Docs:
- plan.md: GMSH Frontal-Delaunay tessellation plan (6 tasks)
- LEARNINGS.md: OCC Polygon3D_s None issue + GCPnts fix
- .gitignore: add backend/core (core dump from root process)

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

frontend/src/api/cad.ts

@@ -98,8 +98,56 @@ export async function generateGltfProduction(cadFileId: string): Promise<Generat
   return res.data
 }
 
+export interface ParsedObjectsResponse {
+  cad_file_id: string
+  original_name: string
+  processing_status: string
+  parsed_objects: {
+    dimensions_mm?: { x: number; y: number; z: number }
+    bbox_center_mm?: { x: number; y: number; z: number }
+    [key: string]: unknown
+  } | null
+}
+
+/** Return the parsed_objects metadata (dimensions, bbox) for a CAD file. */
+export async function getParsedObjects(cadFileId: string): Promise<ParsedObjectsResponse> {
+  const res = await api.get<ParsedObjectsResponse>(`/cad/${cadFileId}/parsed-objects`)
+  return res.data
+}
+
 /** Force-reset a CAD file stuck in 'processing' to 'failed'. */
 export async function resetStuckProcessing(cadFileId: string): Promise<{ status: string; message: string }> {
   const res = await api.post<{ status: string; message: string }>(`/cad/${cadFileId}/reset-stuck`)
   return res.data
 }
+
+// ---------------------------------------------------------------------------
+// Part-material assignment
+// ---------------------------------------------------------------------------
+
+export interface PartMaterialEntry {
+  type: 'library' | 'hex'
+  value: string
+}
+
+export type PartMaterialMap = Record<string, PartMaterialEntry>
+
+interface PartMaterialsResponse {
+  cad_file_id: string
+  part_materials: PartMaterialMap | null
+}
+
+/** Return the saved part-material assignments for a CAD file (empty object if none). */
+export async function getPartMaterials(cadFileId: string): Promise<PartMaterialMap> {
+  const res = await api.get<PartMaterialsResponse>(`/cad/${cadFileId}/part-materials`)
+  return res.data.part_materials ?? {}
+}
+
+/** Replace the part-material assignment map for a CAD file. Returns the updated map. */
+export async function savePartMaterials(
+  cadFileId: string,
+  map: PartMaterialMap,
+): Promise<PartMaterialMap> {
+  const res = await api.put<PartMaterialsResponse>(`/cad/${cadFileId}/part-materials`, map)
+  return res.data.part_materials ?? {}
+}