fix(security): upgrade Go 1.25.7 to 1.26.1 to resolve 4 stdlib vulnerabilities

GO-2026-4602 (os), GO-2026-4601 (net/url), GO-2026-4600 and
GO-2026-4599 (crypto/x509). The crypto/x509 fixes are only
available in go1.26.1+, not backported to go1.25.x.

.github/workflows/backend-ci.yml

@@ -19,7 +19,7 @@ jobs:
           cache: true
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.25.7'
+          go version | grep -q 'go1.26.1'
       - name: Unit tests
         working-directory: backend
         run: make test-unit
@@ -38,7 +38,7 @@ jobs:
           cache: true
       - name: Verify Go version
         run: |
-          go version | grep -q 'go1.25.7'
+          go version | grep -q 'go1.26.1'
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:

.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 'go1.25.7'
+          go version | grep -q 'go1.26.1'
       - name: Run govulncheck
         working-directory: backend
         run: |