fix: escape f-string braces in backend route decorators

{job_id} in f-strings was interpreted as Python variable instead
of FastAPI path parameter. Double braces {{job_id}} produce
literal {job_id} in the route string.

backend/app.py

@@ -105,7 +105,7 @@ def create_job(req: JobSubmissionRequest) -> JobStatusResponse:
         raise HTTPException(status_code=400, detail=str(exc))
 
 
-@app.get(f"{API_PREFIX}/jobs/{job_id}", response_model=JobStatusResponse)
+@app.get(f"{API_PREFIX}/jobs/{{job_id}}", response_model=JobStatusResponse)
 def job_status(job_id: str) -> JobStatusResponse:
     """Get current status of a conversion job."""
     try:
@@ -114,7 +114,7 @@ def job_status(job_id: str) -> JobStatusResponse:
         raise HTTPException(status_code=404, detail=f"Job not found: {job_id}")
 
 
-@app.post(f"{API_PREFIX}/jobs/{job_id}/cleanup", response_model=CleanupResponse)
+@app.post(f"{API_PREFIX}/jobs/{{job_id}}/cleanup", response_model=CleanupResponse)
 def cleanup_job(job_id: str, req: CleanupRequest) -> CleanupResponse:
     """Delete or preserve staging files for a job."""
     try: