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/domains/pipeline/tasks/export_glb.py

@@ -69,7 +69,7 @@ def generate_gltf_geometry_task(self, cad_file_id: str):
     eng.dispose()
 
     linear_deflection = float(sys_settings.get("gltf_preview_linear_deflection", "0.1"))
-    angular_deflection = float(sys_settings.get("gltf_preview_angular_deflection", "0.5"))
+    angular_deflection = float(sys_settings.get("gltf_preview_angular_deflection", "0.1"))
 
     step = _Path(step_path_str)
 
@@ -230,7 +230,7 @@ def generate_gltf_production_task(self, cad_file_id: str, product_id: str | None
 
     smooth_angle = float(sys_settings.get("blender_smooth_angle", "30"))
     prod_linear = float(sys_settings.get("gltf_production_linear_deflection", "0.03"))
-    prod_angular = float(sys_settings.get("gltf_production_angular_deflection", "0.2"))
+    prod_angular = float(sys_settings.get("gltf_production_angular_deflection", "0.05"))
 
     scripts_dir = _Path(_os.environ.get("RENDER_SCRIPTS_DIR", "/render-scripts"))
     occ_script = scripts_dir / "export_step_to_gltf.py"
@@ -239,12 +239,14 @@ def generate_gltf_production_task(self, cad_file_id: str, product_id: str | None
 
     prod_geom_glb = step_path.parent / f"{step_path.stem}_production_geom.glb"
     python_bin = _sys.executable
+    sharp_threshold = float(sys_settings.get("sharp_edge_threshold", "20.0"))
     occ_cmd = [
         python_bin, str(occ_script),
         "--step_path", str(step_path),
         "--output_path", str(prod_geom_glb),
         "--linear_deflection", str(prod_linear),
         "--angular_deflection", str(prod_angular),
+        "--sharp_threshold", str(sharp_threshold),
     ]
     log_task_event(
         self.request.id,