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/media/service.py

@@ -77,12 +77,26 @@ async def delete_media_asset(db: AsyncSession, asset_id: uuid.UUID) -> bool:
 
 
 def get_download_url(asset: MediaAsset) -> str | None:
-    """Return a backend proxy URL so the browser can always download the file."""
-    return f"/api/media/{asset.id}/download"
+    """Return a backend proxy URL so the browser can always download the file.
+
+    Appends ?v={file_size_bytes} as a cache-buster: when a file is regenerated
+    in-place (same asset UUID, new content), the size changes and the URL changes,
+    which triggers a fresh fetch in InlineCadViewer's useEffect.
+    """
+    v = asset.file_size_bytes or 0
+    return f"/api/media/{asset.id}/download?v={v}"
 
 
 def get_thumbnail_url(asset: MediaAsset) -> str | None:
-    """Return CAD thumbnail URL if asset has a cad_file_id."""
+    """Return a no-auth preview URL for the asset.
+
+    Priority:
+    1. For image-type assets (still, thumbnail): the no-auth /thumbnail endpoint.
+    2. For any asset with a cad_file_id: the CAD thumbnail (also no-auth).
+    3. Otherwise None (caller may use _resolve_thumbnails_bulk for fallback).
+    """
+    if asset.asset_type in (MediaAssetType.still, MediaAssetType.thumbnail):
+        return f"/api/media/{asset.id}/thumbnail"
     if asset.cad_file_id:
         return f"/api/cad/{asset.cad_file_id}/thumbnail"
     return None