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/orders.py

@@ -93,6 +93,9 @@ def _build_line_out(line: OrderLine) -> OrderLineOut:
         unit_price=float(line.unit_price) if line.unit_price is not None else None,
         render_position_id=line.render_position_id,
         render_position_name=rp_name,
+        render_log=line.render_log if hasattr(line, 'render_log') else None,
+        render_started_at=line.render_started_at if hasattr(line, 'render_started_at') else None,
+        render_completed_at=line.render_completed_at if hasattr(line, 'render_completed_at') else None,
         notes=line.notes,
         created_at=line.created_at,
         updated_at=line.updated_at,
@@ -384,6 +387,7 @@ async def create_order(
             product_id=line_data.product_id,
             output_type_id=line_data.output_type_id,
             render_position_id=line_data.render_position_id,
+            global_render_position_id=line_data.global_render_position_id,
             gewuenschte_bildnummer=line_data.gewuenschte_bildnummer,
             notes=line_data.notes,
             tenant_id=getattr(user, 'tenant_id', None),
@@ -827,6 +831,7 @@ async def add_order_line(
         product_id=body.product_id,
         output_type_id=body.output_type_id,
         render_position_id=body.render_position_id,
+        global_render_position_id=body.global_render_position_id,
         gewuenschte_bildnummer=body.gewuenschte_bildnummer,
         notes=body.notes,
         tenant_id=getattr(user, 'tenant_id', None),