Fix UnboundLocalError for filament_low in start_job

Variable was only assigned inside a nested if-block; initialise to False
at the top of the filament-logging section so all code paths define it.

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

app/routers/ui.py

@@ -345,6 +345,7 @@ def start_job(job_id: int, printer_id: int = Form(...), db: Session = Depends(ge
     db.add(JobLog(job_id=job_id, message=f"Job started on {printer.name}"))
 
     # Log filament usage against matching AMS roll in FilamentDB
+    filament_low = False
     if job.job_material and job.job_color:
         stl = db.query(StlCache).filter_by(filename=job.file_name).first() if job.file_name else None
         if stl and stl.volume_ccm:
@@ -353,7 +354,6 @@ def start_job(job_id: int, printer_id: int = Form(...), db: Session = Depends(ge
             if roll:
                 note = (f"POPS {job.job_uuid[:8] if job.job_uuid else job.id}: "
                         f"{job.file_name or job.name} on {printer.name}")
-                filament_low = False
                 if log_print_on_roll(roll["roll_id"], weight_g, note):
                     db.add(JobLog(job_id=job_id, message=(
                         f"FilamentDB: logged {weight_g} g against roll #{roll['roll_id']} "