feat(postgres): support configurable schema

[thomasjiangcy]

Summary

• add optional to options (default )
• route Postgres runtime queries through schema-aware identifiers instead of hardcoded
• validate schema names before running migrations/queries and harden migration SQL interpolation
• update docs for Postgres/production to describe configurable schema usage
• add tests for custom schema behavior and invalid schema names

Notes

• this keeps existing behavior unchanged when is omitted
• motivated by Postgres - allow custom schema #291

Closes #291

[thomasjiangcy]

Superseded by #293 (same changes on a signed-commit branch).