CI: Add PostgreSQL integration tests to CI matrix

[renecannao]

Add PostgreSQL integration tests (providers/postgresql/integration_test.go) to the CI test matrix. Requires PostgreSQL deb caching (#51) to be in place.

Part of Phase 3 — PostgreSQL provider.

[coderabbitai]

🔗 Related PRs

#44 - Extend Provider interface with lifecycle methods [merged]

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