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.

backend/.gitignore

@@ -0,0 +1,9 @@
+*.png
+__pycache__
+app.egg-info
+*.pyc
+.mypy_cache
+.coverage
+htmlcov
+.cache
+.venv