Site blueprint: a Collections field for barakoPress's configured lists

barakoPress reads a tenant's collections (content type, route, field mapping, filters, sort, colour by option) from a `Collections` setting on the tenant's `site` entry (BaryoDev/barakoPress#5, PR BaryoDev/barakoPress#39). The built-in `site` blueprint (#793) has no such field, so each tenant would have to add it by hand before a site kit (#721) can configure its lists.

## Change

- Add `Collections` (json) to `barakoCMS/Blueprints/site.json`, documented in `docs/site-settings.md` with the shape barakoPress reads and one example (an events list filtered by a choice field and coloured through `OptionColors`).
- Applying the blueprint to a tenant that already has a `site` type with the field is a no-op; a tenant without it gains it through the add-field path.

## Done when

- A fresh tenant with the `site` blueprint has `Collections`, `SiteBlueprintTests` covers it, and the docs example matches what barakoPress parses.

## Where it lives

Core: the built-in blueprint and its docs.

Start with barakoCMS/Blueprints/site.json and docs/site-settings.md, then inspect SiteBlueprintTests for the existing blueprint assertions and add-field behavior. Done means a fresh tenant receives Collections, an existing field is unchanged, the test covers it, and the documentation example matches what barakoPress parses.

Written by the indexing model from the issue text.