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/main.py

@@ -17,7 +17,7 @@ from app.domains.orders.router import orders_router, order_items_router
 from app.domains.admin.router import admin_router, analytics_router, worker_router
 from app.domains.products.router import products_router, cad_router
 from app.domains.materials.router import router as materials_router
-from app.domains.rendering.router import render_templates_router, output_types_router
+from app.domains.rendering.router import render_templates_router, output_types_router, global_render_positions_router
 from app.domains.notifications.router import router as notifications_router
 from app.domains.billing.router import pricing_router, invoice_router
 from app.domains.tenants.router import router as tenants_router
@@ -94,6 +94,7 @@ app.include_router(media_router)
 app.include_router(asset_libraries_router, prefix="/api")
 app.include_router(dashboard_router, prefix="/api")
 app.include_router(task_logs_router, prefix="/api")
+app.include_router(global_render_positions_router, prefix="/api")
 
 
 @app.get("/health")