Make printer optional on job creation — assigned when job starts

Migration 0005 makes print_jobs.printer_id nullable.

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

app/routers/ui.py

@@ -141,7 +141,7 @@ def jobs_page(request: Request, q: str = "", db: Session = Depends(get_db)):
 
 @router.post("/jobs")
 async def create_job(
-    printer_id: int = Form(...),
+    printer_id: str = Form(""),
     customer: str = Form(""),
     filament_id: str = Form(""),
     notes: str = Form(""),
@@ -160,7 +160,7 @@ async def create_job(
         job_uuid=str(_uuid.uuid4()),
         name=Path(filename).stem if filename else (customer.strip() or "Unnamed"),
         file_name=filename,
-        printer_id=printer_id,
+        printer_id=int(printer_id) if printer_id else None,
         filament_id=int(filament_id) if filament_id else None,
         customer=customer.strip() or None,
         notes=notes.strip() or None,