ci: run integration tests in CI (Postgres-backed, no silent skip)

[segidev]

Problem

The integration test suite (//go:build integration, internal/store/integration_test.go) does not run in CI. The test job runs go test -count=1 ./... with no -tags=integration and no Postgres, so the build-tagged tests are never compiled or executed. They only run locally via make test-integration.

Introduced alongside the harness in #19 (v0.19.0): intake-append, block-freeze lifecycle, decline repoint, active-version resolution, and full-delete coverage currently has no CI gate.

Proposed fix

Add an integration job to .github/workflows/ci.yml:

• Provision Postgres (GH services: or the existing make test-integration which brings up docker-compose.test.yml).
• Set REQUIRE_TEST_DB=true so a missing DB is fatal, never a silent skip.
• Run make test-integration.

Acceptance

• CI runs the //go:build integration tests on every PR to main.
• A missing/unreachable DB fails the job (no skip).
• Existing unit/lint/security/sandbox jobs unaffected.

Tracking

ClickUp: https://app.clickup.com/t/86ca2uh21