Improve Databases and Tables TypeScript types to support iteration

## Problem

`Databases` and `Tables` types in `resources/Table.ts` are plain-object maps, not `Map` instances or arrays, so they don't support standard iteration (`.forEach`, `for...of`, spread). TypeScript developers expect collection types to be iterable.

## Proposed

Convert to `Map` / `Map` or add iterator support so callers can iterate naturally without using `Object.entries()`.

🤖 Filed by Claude on behalf of Kris.