Analyse STL/3MF on upload: bounding box + volume, cached in stl_cache

- StlCache model: filename, cube_x/y/z (mm), volume_ccm
- Migration 0007: stl_cache table
- app/stl_analysis.py: trimesh-based analyse() + get_cache_map()
  - Runs immediately after upload in both /jobs and /brackets/{id}/jobs
  - Skips .gcode (no geometry), caches per filename, idempotent
- Library selector in both modals now shows dimensions and volume
- requirements: trimesh==4.5.3

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

requirements.txt

@@ -6,3 +6,4 @@ pymysql==1.1.1
 cryptography==44.0.3
 jinja2==3.1.4
 python-multipart==0.0.20
+trimesh==4.5.3