Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/utils/item.py

@@ -0,0 +1,16 @@
+from sqlmodel import Session
+
+from app import crud
+from app.models import Item, ItemCreate
+from tests.utils.user import create_random_user
+from tests.utils.utils import random_lower_string
+
+
+def create_random_item(db: Session) -> Item:
+    user = create_random_user(db)
+    owner_id = user.id
+    assert owner_id is not None
+    title = random_lower_string()
+    description = random_lower_string()
+    item_in = ItemCreate(title=title, description=description)
+    return crud.create_item(session=db, item_in=item_in, owner_id=owner_id)