CI: PostgreSQL version matrix (15, 16, 17)

[renecannao]

Summary

PostgreSQL support was added recently and currently only tests against PostgreSQL 17. Since orchestrator uses pg_stat_wal_receiver, pg_stat_replication, and pg_promote() which have evolved across versions, we should test against multiple PostgreSQL versions.

Proposal

• Parameterize the functional test Docker Compose to accept a PG_IMAGE variable
• Add a GitHub Actions matrix strategy to run PostgreSQL tests against: postgres:15, postgres:16, postgres:17
• Ensure init scripts and orchestrator queries are compatible across versions

Acceptance Criteria

• PostgreSQL functional tests pass on PG 15, 16, and 17
• Matrix runs in parallel in CI
• Remove continue-on-error from PostgreSQL test step if all versions pass