fix(ci): upgrade golangci-lint v2.7 to v2.11 for Go 1.26 compatibility

golangci-lint v2.7 was built with Go 1.25 and cannot lint Go 1.26
targets. v2.8+ added Go 1.26 support.

.github/workflows/backend-ci.yml

@@ -42,6 +42,6 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.7
+          version: v2.11
           args: --timeout=30m
           working-directory: backend
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