fix(api): group create read product from multipart form (bug: 'provide product info' when no file attached)

The GroupBuilder sends the create form as multipart/form-data (FormData). The code
decided JSON vs form by checking  — when no file was attached,
request.files was empty/falsy, so it tried get_json() on a multipart body and lost the
product field -> 400 'provide product info' even though the user filled the product name.

Fix: branch on the Content-Type (multipart/form-data -> request.form) instead of
request.files. Added a regression test (multipart create with product only -> 201).
Verified live: multipart product-only create now returns 201.

backend/app/api/group_routes.py

@@ -88,7 +88,10 @@ def create_group():
             file.save(dest)
             saved_files.append(dest.name)
 
-    data = request.form.to_dict() if request.files else (request.get_json(silent=True) or {})
+    if request.content_type and "multipart/form-data" in request.content_type:
+        data = request.form.to_dict()
+    else:
+        data = request.get_json(silent=True) or {}
 
     from ..services.file_parser import parse_document