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/scripts/test_m1.py

@@ -47,6 +47,18 @@ def main():
     assert r.get_json()["group"]["status"] == "draft"
     print("[ok] group created (draft)")
 
+    # Regression: multipart (like the frontend FormData) with product field but NO file
+    # attached must still create the group (was 400 'provide product info' before fix).
+    r = client.post(
+        "/api/groups",
+        data={"product": "Multipart Product"},
+        content_type="multipart/form-data",
+        headers=H,
+    )
+    assert r.status_code == 201, f"multipart create failed: {r.get_json()}"
+    assert r.get_json()["group"]["title"] == "Multipart Product"
+    print("[ok] group created via multipart (product only) — regression fixed")
+
     # analyze should fail cleanly (LLM None)
     r = client.post(f"/api/groups/{gid}/analyze", headers=H)
     assert r.status_code == 500, r.get_json()
@@ -54,8 +66,9 @@ def main():
 
     # list groups as admin
     r = client.get("/api/groups", headers=H)
-    assert r.status_code == 200 and len(r.get_json()["groups"]) == 1
-    print("[ok] admin lists 1 group")
+    assert r.status_code == 200 and len(r.get_json()["groups"]) >= 1
+    assert all(g["status"] == "draft" for g in r.get_json()["groups"])
+    print("[ok] admin lists groups")
 
     # personas empty until analyze
     r = client.get(f"/api/groups/{gid}", headers=H)