feat(page): add typed keyset pagination for Bob

Add versioned typed cursors, composable native query mods, bidirectional page assembly and opt-in authorized counts. Cover SQLite and PostgreSQL outcomes, strict cursor boundaries and a runnable example; require PostgreSQL service tests in CI.

Refs: #6

.gitea/workflows/ci.yml

@@ -41,3 +41,23 @@ jobs:
           distribution: goreleaser
           version: v2.19.0
           args: release --snapshot --clean
+  postgres:
+    runs-on: ubuntu-latest
+    container: golang:1.27.1
+    services:
+      postgres:
+        image: postgres:17-alpine
+        env:
+          POSTGRES_PASSWORD: page-test
+          POSTGRES_DB: page_test
+        options: >-
+          --health-cmd "pg_isready -U postgres -d page_test"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 12
+    env:
+      PAGE_POSTGRES_DSN: postgres://postgres:page-test@postgres:5432/page_test?sslmode=disable
+      PAGE_REQUIRE_POSTGRES: "1"
+    steps:
+      - uses: actions/checkout@v7.0.1
+      - run: go test -race ./page