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>

frontend/nginx.conf

@@ -0,0 +1,11 @@
+server {
+  listen 80;
+
+  location / {
+    root /usr/share/nginx/html;
+    index index.html index.htm;
+    try_files $uri /index.html =404;
+  }
+
+  include /etc/nginx/extra-conf.d/*.conf;
+}