fix(gallery): confine replacement image path (#4285)

routes/gallery_routes.py

@@ -232,8 +232,6 @@ def setup_gallery_routes() -> APIRouter:
     @router.post("/api/gallery/{image_id}/replace")
     async def gallery_replace(request: Request, image_id: str):
         """Replace an existing gallery image file with a new one."""
-        from pathlib import Path
-
         user = get_current_user(request)
         db = SessionLocal()
         try:
@@ -249,9 +247,8 @@ def setup_gallery_routes() -> APIRouter:
                 raise HTTPException(400, "No image provided")
 
             content = await read_upload_limited(file, GALLERY_UPLOAD_MAX_BYTES, "Gallery replacement")
-            img_dir = Path(GENERATED_IMAGES_DIR)
-            img_dir.mkdir(parents=True, exist_ok=True)
-            img_path = img_dir / _sanitize_gallery_filename(img.filename)
+            GALLERY_IMAGE_DIR.mkdir(parents=True, exist_ok=True)
+            img_path = _gallery_image_path(img.filename)
             img_path.write_bytes(content)
 
             # Refresh dimensions in case the editor resized the canvas.