PostgreSQL 19 Support

[Dev-iL]

Caution

Do not undraft/merge until PG19 reaches general availability!

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Adds 19beta1 to ALLOWED_POSTGRES_VERSIONS / CURRENT_POSTGRES_VERSIONS in dev/breeze/src/airflow_breeze/global_constants.py so CI exercises PostgreSQL 19 during its beta cycle, surfacing any compatibility issues ahead of GA. dev/breeze/tests/test_selective_checks.py and the breeze doc image/output snapshots are updated to match.

This PR exists to validate CI compatibility with PostgreSQL 19 while it is still in beta. It intentionally targets the 19beta1 image tag rather than a GA release, and per the TODO left in global_constants.py, "14" should also be dropped once it reaches EOL (2026-11-13) and the default shifted.

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Generated-by: Claude Code (Sonnet 5) following the guidelines

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