Add cancel job action

POST /jobs/{id}/cancel sets status → cancelled, frees printer if printing,
logs 'Job cancelled'. Button visible in all non-terminal states (queued, printing).

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

app/routers/ui.py

@@ -354,6 +354,20 @@ def start_job(job_id: int, printer_id: int = Form(...), db: Session = Depends(ge
     return RedirectResponse(url=f"/jobs/{job_id}", status_code=HTTP_303_SEE_OTHER)
 
 
+@router.post("/jobs/{job_id}/cancel")
+def cancel_job(job_id: int, db: Session = Depends(get_db)):
+    from fastapi import HTTPException
+    job = db.query(PrintJob).filter(PrintJob.id == job_id).first()
+    if not job:
+        raise HTTPException(404, "Job not found")
+    if job.printer and job.status == JobStatus.printing:
+        job.printer.status = PrinterStatus.idle
+    job.status = JobStatus.cancelled
+    db.add(JobLog(job_id=job_id, message="Job cancelled"))
+    db.commit()
+    return RedirectResponse(url=f"/jobs/{job_id}", status_code=HTTP_303_SEE_OTHER)
+
+
 @router.post("/jobs/{job_id}/stop")
 def stop_job(job_id: int, db: Session = Depends(get_db)):
     from fastapi import HTTPException