fix(schema): avoid rebuilding current indexes - #830

Summary

• inspect each redefined index in PostgreSQL's catalog before dropping it
• leave already-current indexes in place when the offline converge script is re-applied
• still rebuild stale definitions so IF NOT EXISTS cannot preserve an outdated index

Closes #828.

Testing

• uv sync --all-extras --frozen && uv run ruff check . && uv run ruff format . --check && uv run lint-imports && uv run mypy . && uv run pytest passed (1262 passed, 11 skipped)
• Added PostgreSQL integration coverage showing current indexes keep their relfilenode and a stale index is restored to the declared definition

Checklist

• I have read the Contributing Guide
• I have added or updated tests
• I have updated documentation if necessary (no documentation change is needed for this behavior fix)