fix(backend): harden postgres id default repair and CI coverage

Run Postgres id-sequence repair on every startup migration and add contract coverage plus a GitHub Actions Postgres job to catch schema-drift regressions before release.

.github/workflows/ci-build.yml

@@ -48,6 +48,43 @@ jobs:
       - name: Build
         run: go build -v ./...
 
+  backend-postgres-contract:
+    name: Go Backend PostgreSQL Contract
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: postgres:17
+        env:
+          POSTGRES_USER: flux_test
+          POSTGRES_PASSWORD: flux_test_pass
+          POSTGRES_DB: flux_test
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U flux_test -d flux_test"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 10
+    defaults:
+      run:
+        working-directory: go-backend
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.23'
+          cache-dependency-path: go-backend/go.sum
+
+      - name: Download dependencies
+        run: go mod download
+
+      - name: Run PostgreSQL contract test
+        env:
+          FLVX_POSTGRES_TEST_DSN: 'postgres://flux_test:flux_test_pass@127.0.0.1:5432/flux_test?sslmode=disable'
+        run: go test ./tests/contract -run TestPostgresNodeCreateRepairsMissingIDDefaultContract -count=1
+
   agent:
     name: Build Agent
     runs-on: ubuntu-latest