Add file download for job models

- GET /files/{filename}: serves file from STL_UPLOAD_DIR with path-traversal guard
- Download button in File & Geometry card header (always visible when file attached)
- Download button also appears at the top of the Start Job modal

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

app/routers/ui.py

@@ -4,7 +4,7 @@ from datetime import datetime, timedelta
 from pathlib import Path
 
 from fastapi import APIRouter, Depends, File, Form, Request, UploadFile
-from fastapi.responses import HTMLResponse, RedirectResponse
+from fastapi.responses import FileResponse, HTMLResponse, RedirectResponse
 from fastapi.templating import Jinja2Templates
 from sqlalchemy import case, or_
 from sqlalchemy.orm import Session
@@ -297,6 +297,18 @@ def edit_job(
     return RedirectResponse(url=f"/jobs/{job_id}", status_code=HTTP_303_SEE_OTHER)
 
 
+@router.get("/files/{filename}")
+def download_file(filename: str):
+    from fastapi import HTTPException
+    stl_dir = get_stl_dir()
+    path = (stl_dir / filename).resolve()
+    if not str(path).startswith(str(stl_dir.resolve())):
+        raise HTTPException(400, "Invalid filename")
+    if not path.exists():
+        raise HTTPException(404, "File not found")
+    return FileResponse(path, filename=filename, media_type="application/octet-stream")
+
+
 @router.post("/jobs/{job_id}/start")
 def start_job(job_id: int, printer_id: int = Form(...), db: Session = Depends(get_db)):
     from fastapi import HTTPException