chore: use ruff to lint and auto-format

backend/.vscode/extensions.json

@@ -0,0 +1,3 @@
+{
+  "recommendations": ["charliermarsh.ruff"]
+}

backend/.vscode/settings.json

@@ -0,0 +1,10 @@
+{
+  "[python]": {
+    "editor.formatOnSave": true,
+    "editor.codeActionsOnSave": {
+      "source.fixAll": "explicit",
+      "source.organizeImports": "explicit"
+    },
+    "editor.defaultFormatter": "charliermarsh.ruff"
+  }
+}