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/stl_analysis.py

@@ -8,6 +8,7 @@ from app.models.stl_cache import StlCache
 logger = logging.getLogger("pops")
 
 _ANALYSABLE = {".stl", ".3mf"}
+_in_progress: set[str] = set()
 
 
 def analyse(filename: str, db: Session) -> StlCache | None:
@@ -45,6 +46,26 @@ def analyse(filename: str, db: Session) -> StlCache | None:
         return None
 
 
+def analyse_in_background(filename: str) -> None:
+    """Run analyse() in a background task with its own DB session."""
+    if filename in _in_progress:
+        return
+    _in_progress.add(filename)
+    try:
+        from app.db import SessionLocal
+        db = SessionLocal()
+        try:
+            analyse(filename, db)
+        finally:
+            db.close()
+    finally:
+        _in_progress.discard(filename)
+
+
+def is_computing(filename: str) -> bool:
+    return filename in _in_progress
+
+
 def get_cache_map(filenames: list[str], db: Session) -> dict[str, StlCache]:
     """Bulk-fetch cached entries for a list of filenames."""
     rows = db.query(StlCache).filter(StlCache.filename.in_(filenames)).all()