Name uploaded and downloaded files J-<id>.<ext>

Files added through the upload pipeline and Download to Library are
renamed to their J-ID right after indexing, so every new library file is
traceable by its identifier (scanned files keep their existing names).
index_file now returns the created location so callers can rename it;
the location record is updated to the new path.

backend/apps/library/services.py

@@ -41,7 +41,7 @@ def compute_md5(path):
 def index_file(path, folder):
     """Index one file into MediaItem/MediaLocation.
 
-    Returns (item, created_item, created_location).
+    Returns (item, created_item, location, created_location).
     """
     path = Path(path)
     folder = Path(folder)
@@ -61,7 +61,25 @@ def index_file(path, folder):
             "mtime": stat.st_mtime,
         },
     )
-    return item, created_item, created_location
+    return item, created_item, location, created_location
+
+
+def rename_location_to_j_id(item, location):
+    """Name a freshly indexed copy J-<id>.<ext> inside its own folder."""
+    path = Path(location.path)
+    if not path.exists():
+        return location
+    if path.stem == f"J-{item.id}":
+        return location
+    target = unique_destination(path.parent, f"J-{item.id}{path.suffix}")
+    path.rename(target)
+    parent = Path(location.rel_path).parent
+    location.path = str(target)
+    location.rel_path = (
+        str(parent / target.name) if str(parent) != "." else target.name
+    )
+    location.save(update_fields=["path", "rel_path"])
+    return location
 
 
 def parse_tags(raw):