ci/cd: run linter only on backend changes

.github/workflows/backend-linter.yml

@@ -2,11 +2,14 @@ name: Run Backend Linter
 
 on:
   push:
-        branches:
-            - main
+    branches: [main]
+    paths:
+      - "backend/**"
   pull_request:
-        branches:
-            - main
+    branches: [main]
+    paths:
+      - "backend/**"
+
 permissions:
   # Required: allow read access to the content for analysis.
   contents: read