Background STL analysis with computing spinner; manual geometry override

- analyse_in_background() runs in FastAPI BackgroundTasks with its own
  DB session; _in_progress set prevents duplicate concurrent analyses
- Job detail shows a spinner + auto-reloads every 3s while computing
- 'Compute now' button also triggers background analysis
- StlCache.manually_set flag (migration 0011): entries marked manual are
  never overwritten by auto-analysis
- 'Override values' collapsible form on job detail lets user set
  bounding box and volume directly from slicer data

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

app/models/stl_cache.py

@@ -1,6 +1,6 @@
 from datetime import datetime
 
-from sqlalchemy import DateTime, Float, String, func
+from sqlalchemy import Boolean, DateTime, Float, String, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.models.base import Base
@@ -15,4 +15,5 @@ class StlCache(Base):
     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
+    manually_set: Mapped[bool] = mapped_column(Boolean, default=False, server_default="0")
     created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())