Fix 3MF analysis: concatenate scene geometries instead of force="mesh"

trimesh loads 3MF files as a Scene; force="mesh" fails on those.
Detect Scene and concatenate all geometries before computing extents/volume.

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

app/stl_analysis.py

@@ -23,7 +23,11 @@ def analyse(filename: str, db: Session) -> StlCache | None:
 
     try:
         import trimesh
-        mesh = trimesh.load(str(filepath), force="mesh")
+        loaded = trimesh.load(str(filepath))
+        if isinstance(loaded, trimesh.Scene):
+            mesh = trimesh.util.concatenate(list(loaded.geometry.values()))
+        else:
+            mesh = loaded
         extents = mesh.extents          # [size_x, size_y, size_z] in mm
         volume_mm3 = abs(mesh.volume)   # mm³ (negative for inverted normals)
         entry = StlCache(