fix: bump Go from 1.26.1 to 1.26.2 to resolve 6 stdlib CVEs

Fixes GO-2026-4947, GO-2026-4946, GO-2026-4870, GO-2026-4869,
GO-2026-4866, GO-2026-4865 in crypto/x509, crypto/tls, archive/tar,
and html/template.

.github/workflows/security-scan.yml

@@ -23,7 +23,7 @@ jobs:
           cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.26.1'
+          go version | grep -q 'go1.26.2'
       - name: Run govulncheck
         working-directory: backend
         run: |