feat: add databases command

[TacoRocket]

Summary

• add a SQL-first databases command for Azure SQL server posture and visible user-database inventory
• add fixture, schema, golden, help, CLI smoke, and terminal UX coverage for the new slice
• park relational depth and broader non-relational database follow-on work in the future roadmap

Validation

• python3 -m ruff check src tests scripts
• PYTHONPATH=src python3 scripts/generate_schemas.py
• PYTHONPATH=src python3 -m pytest tests/test_collectors.py tests/test_golden_outputs.py tests/test_contract_schemas.py tests/test_cli_smoke.py tests/test_help.py tests/test_terminal_ux.py tests/test_models.py
• PYTHONPATH=src python3 -m pytest