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>

backend/app/api/routers/products.py

@@ -76,6 +76,7 @@ def _product_out(product: Product, priority: list[str] | None = None) -> Product
     out.processing_status = product.processing_status
     out.cad_parsed_objects = product.cad_parsed_objects
     out.cad_mesh_attributes = product.cad_file.mesh_attributes if product.cad_file else None
+    out.cad_render_log = product.cad_file.render_log if product.cad_file else None
     out.render_image_url = _best_render_url(product, priority or ["latest_render", "cad_thumbnail"])
     return out
 
@@ -662,6 +663,8 @@ async def get_product_renders(
         .options(
             joinedload(OrderLine.output_type),
             joinedload(OrderLine.order),
+            joinedload(OrderLine.render_position),
+            joinedload(OrderLine.global_render_position),
         )
         .where(
             OrderLine.product_id == product_id,
@@ -681,6 +684,11 @@ async def get_product_renders(
         if disk is None or not disk.exists():
             continue
         ext = Path(url).suffix.lower()
+        position_name = (
+            line.render_position.name if line.render_position
+            else line.global_render_position.name if line.global_render_position
+            else None
+        )
         renders.append({
             "order_line_id": str(line.id),
             "order_number": line.order.order_number if line.order else None,
@@ -689,6 +697,7 @@ async def get_product_renders(
             "is_video": ext in VIDEO_EXTENSIONS,
             "render_backend": line.render_backend_used,
             "completed_at": line.render_completed_at.isoformat() if line.render_completed_at else None,
+            "render_position_name": position_name,
         })
     return renders