test: expand integration test coverage across backends

[kcenon]

What

Only 3 integration tests exist today. Expand to at least 15 scenarios covering transactions, concurrency, failure modes, and backend parity across PostgreSQL and SQLite.

• Current: 3 integration tests
• Expected: >=15 scenarios, parameterized per backend
• Scope: tests/integration/**

Why

• Integration gaps cause regressions when switching backends
• ACID correctness claims need multi-backend proof

How

Technical Approach

1. Draft a scenario matrix: CRUD, transactions (commit/rollback), isolation levels, connection loss, schema migration
2. Use testcontainers-cpp (or docker-compose) for PostgreSQL
3. Parameterize GTest to run each case on both backends
4. Add workflow .github/workflows/integration.yml

Acceptance Criteria

• 15+ parameterized integration tests
• PostgreSQL and SQLite both covered
• Integration workflow runs on PRs
• Documented scenario matrix in docs/testing/SCENARIOS.md

[kcenon]

Part of kcenon/common_system#647 (Epic: Ecosystem-wide test coverage uplift)

[kcenon]

Implementation PR: #571 - merged.