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/models/__init__.py

@@ -10,7 +10,7 @@ from app.domains.products.models import CadFile, Product
 from app.domains.orders.models import Order, OrderItem, OrderLine
 from app.domains.notifications.models import AuditLog
 from app.domains.billing.models import PricingTier
-from app.domains.rendering.models import OutputType, RenderTemplate, ProductRenderPosition, WorkflowDefinition, WorkflowRun, WorkflowNodeResult
+from app.domains.rendering.models import OutputType, RenderTemplate, ProductRenderPosition, GlobalRenderPosition, WorkflowDefinition, WorkflowRun, WorkflowNodeResult
 from app.domains.materials.models import Material, MaterialAlias, AssetLibrary
 from app.domains.media.models import MediaAsset, MediaAssetType
 from app.domains.admin.models import DashboardConfig
@@ -21,7 +21,7 @@ from app.models.worker_config import WorkerConfig
 
 __all__ = [
     "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
-    "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
+    "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition", "GlobalRenderPosition",
     "WorkflowDefinition", "WorkflowRun", "WorkflowNodeResult",
     "Material", "MaterialAlias", "AssetLibrary", "MediaAsset", "MediaAssetType", "SystemSetting",
     "DashboardConfig", "WorkerConfig",