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/scripts/test_backend_pre_start.py

@@ -0,0 +1,33 @@
+from unittest.mock import MagicMock, patch
+
+from sqlmodel import select
+
+from app.backend_pre_start import init, logger
+
+
+def test_init_successful_connection() -> None:
+    engine_mock = MagicMock()
+
+    session_mock = MagicMock()
+    session_mock.__enter__.return_value = session_mock
+
+    select1 = select(1)
+
+    with (
+        patch("app.backend_pre_start.Session", return_value=session_mock),
+        patch("app.backend_pre_start.select", return_value=select1),
+        patch.object(logger, "info"),
+        patch.object(logger, "error"),
+        patch.object(logger, "warn"),
+    ):
+        try:
+            init(engine_mock)
+            connection_successful = True
+        except Exception:
+            connection_successful = False
+
+        assert connection_successful, (
+            "The database connection should be successful and not raise an exception."
+        )
+
+        session_mock.exec.assert_called_once_with(select1)