Backend integration tests with Testcontainers (Postgres + Iceberg REST)

[adelfardi]

Backend currently has a single unit test. Add Testcontainers-based integration tests spinning up Postgres + an Iceberg REST catalog, covering:

• catalog CRUD (incl. the persisted vendor field)
• namespace + table create
• statistics / storage overview
• executions search

Surface a JaCoCo coverage report in CI.