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/render_thumbnail.py

@@ -95,6 +95,38 @@ def render_step_thumbnail(self, cad_file_id: str):
     except Exception:
         logger.exception(f"bbox extraction failed for {cad_file_id} (non-fatal)")
 
+    # Extract sharp edge topology (PCurve-based) if not already present.
+    # This runs on render-worker which has OCP (cadquery's OCC fork).
+    try:
+        from sqlalchemy import create_engine
+        from sqlalchemy.orm import Session
+        from app.config import settings as _cfg3
+        from app.models.cad_file import CadFile as _CadFile3
+        from app.services.step_processor import extract_mesh_edge_data
+
+        _sync_url3 = _cfg3.database_url.replace("+asyncpg", "")
+        _eng3 = create_engine(_sync_url3)
+        with Session(_eng3) as _sess3:
+            _cad3 = _sess3.get(_CadFile3, cad_file_id)
+            _attrs = _cad3.mesh_attributes or {} if _cad3 else {}
+            _step_path3 = _cad3.stored_path if _cad3 else None
+        _eng3.dispose()
+
+        if _step_path3 and "sharp_edge_pairs" not in _attrs:
+            edge_data = extract_mesh_edge_data(_step_path3)
+            if edge_data:
+                _eng3 = create_engine(_sync_url3)
+                with Session(_eng3) as _sess3:
+                    _cad3 = _sess3.get(_CadFile3, cad_file_id)
+                    if _cad3:
+                        _cad3.mesh_attributes = {**(_cad3.mesh_attributes or {}), **edge_data}
+                        _sess3.commit()
+                        n_pairs = len(edge_data.get("sharp_edge_pairs", []))
+                        logger.info(f"Sharp edge data extracted for {cad_file_id}: {n_pairs} sharp edges")
+                _eng3.dispose()
+    except Exception:
+        logger.exception(f"Sharp edge extraction failed for {cad_file_id} (non-fatal)")
+
     # Auto-populate materials now that parsed_objects are available
     try:
         from app.domains.pipeline.tasks.extract_metadata import _auto_populate_materials_for_cad