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>

app/models/stl_cache.py

@@ -0,0 +1,18 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, Float, String, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base
+
+
+class StlCache(Base):
+    __tablename__ = "stl_cache"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    filename: Mapped[str] = mapped_column(String(255), unique=True)
+    cube_x: Mapped[float | None] = mapped_column(Float)   # mm
+    cube_y: Mapped[float | None] = mapped_column(Float)   # mm
+    cube_z: Mapped[float | None] = mapped_column(Float)   # mm
+    volume_ccm: Mapped[float | None] = mapped_column(Float)  # cm³
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())