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>

alembic/versions/0011_stl_cache_manually_set.py

@@ -0,0 +1,21 @@
+"""stl_cache manually_set flag
+
+Revision ID: 0011
+Revises: 0010
+Create Date: 2026-06-19
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0011"
+down_revision = "0010"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("stl_cache", sa.Column("manually_set", sa.Boolean(), server_default="0", nullable=False))
+
+
+def downgrade():
+    op.drop_column("stl_cache", "manually_set")