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/scripts/prestart.sh

@@ -0,0 +1,13 @@
+#! /usr/bin/env bash
+
+set -e
+set -x
+
+# Let the DB start
+python app/backend_pre_start.py
+
+# Run migrations
+alembic upgrade head
+
+# Create initial data in DB
+python app/initial_data.py