Update admin theme/layout and refresh README details.

Align the project baseline with the latest admin interface styling and layout structure while documenting setup and usage updates in README.

apps/api/scripts/export_openapi.py

@@ -0,0 +1,14 @@
+from pathlib import Path
+import json
+
+from app.main import app
+
+
+def main() -> None:
+    schema = app.openapi()
+    output = Path(__file__).resolve().parent.parent / "openapi.json"
+    output.write_text(json.dumps(schema, indent=2), encoding="utf-8")
+
+
+if __name__ == "__main__":
+    main()