feat(P3): add GMSH Frontal-Delaunay tessellation engine

Introduces GMSH as an alternative to OCC BRepMesh for STEP→GLB tessellation.
GMSH produces conforming meshes that eliminate fan triangles at cylinder seam
edges — a structural limitation of OCC BRepMesh that cannot be fixed via
deflection parameters.

Changes:
- render-worker/Dockerfile: install gmsh>=4.15.0 + libglu1-mesa + libxft2
- export_step_to_gltf.py: --tessellation_engine occ|gmsh CLI arg +
  _tessellate_with_gmsh() using BRep→GMSH→Poly_Triangulation write-back
- admin.py: tessellation_engine setting (SETTINGS_DEFAULTS, SettingsOut,
  SettingsUpdate, validation)
- export_glb.py: pass tessellation_engine to export_step_to_gltf.py CLI in
  both geometry and production GLB tasks
- Admin.tsx: radio button UI for OCC vs GMSH selection

Tested: 121 faces meshed, 0 BRepMesh fallback, 649K triangles on sample part.
Clean seam edges for UV unwrap — GMSH respects B-rep periodic face boundaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routers/admin.py

@@ -41,7 +41,8 @@ SETTINGS_DEFAULTS: dict[str, str] = {
     "smtp_user":                       "",
     "smtp_password":                   "",
     "smtp_from_address":               "",
-    # glTF tessellation quality (OCC BRepMesh)
+    # glTF tessellation quality
+    "tessellation_engine":                 "occ",   # "occ" | "gmsh" — tessellation backend
     "gltf_preview_linear_deflection":      "0.1",   # mm — geometry GLB for viewer
     "gltf_preview_angular_deflection":     "0.1",   # rad — Standard preset
     "gltf_production_linear_deflection":   "0.03",  # mm — production GLB
@@ -87,6 +88,7 @@ class SettingsOut(BaseModel):
     gltf_material_quality: str = "pbr_colors"
     gltf_pbr_roughness: float = 0.4
     gltf_pbr_metallic: float = 0.6
+    tessellation_engine: str = "occ"
 
 
 class SettingsUpdate(BaseModel):
@@ -119,6 +121,7 @@ class SettingsUpdate(BaseModel):
     gltf_material_quality: str | None = None
     gltf_pbr_roughness: float | None = None
     gltf_pbr_metallic: float | None = None
+    tessellation_engine: str | None = None
 
 
 @router.get("/users", response_model=list[UserOut])
@@ -237,6 +240,7 @@ def _settings_to_out(raw: dict[str, str]) -> SettingsOut:
         gltf_material_quality=raw.get("gltf_material_quality", "pbr_colors"),
         gltf_pbr_roughness=float(raw.get("gltf_pbr_roughness", "0.4")),
         gltf_pbr_metallic=float(raw.get("gltf_pbr_metallic", "0.6")),
+        tessellation_engine=raw.get("tessellation_engine", "occ"),
     )
 
 
@@ -361,6 +365,10 @@ async def update_settings(
         if not (0.05 <= body.gltf_production_angular_deflection <= 1.5):
             raise HTTPException(400, detail="gltf_production_angular_deflection must be 0.05–1.5 rad")
         updates["gltf_production_angular_deflection"] = str(body.gltf_production_angular_deflection)
+    if body.tessellation_engine is not None:
+        if body.tessellation_engine not in {"occ", "gmsh"}:
+            raise HTTPException(400, detail="tessellation_engine must be 'occ' or 'gmsh'")
+        updates["tessellation_engine"] = body.tessellation_engine
 
     for k, v in updates.items():
         await _save_setting(db, k, v)