Domain-level frontend settings

Just like #2349, let's add admin-level frontend settings API. This settings will cover domain/site-specific theme customizations (brand colors, font, logo, etc.) and default localization settings for new users (timezone, language, etc.). This could be also reused across different add-ons like FastTrack.

- [ ] Add a `config-domain-frontend` json schema definition, placing it under `src/ai/backend/manager/models/schema`.\* For logo, let's just accept an arbitrary URL.

- [ ] Add a `frontend_settings` column to the `domains` table.\* Fill the default settings when migrating the database.

- [ ] Add a frontend configuration API set to the manager:\* `POST|GET /config/frontend/{domain_id`} (write/read config; only superadmins can write)
- `GET /config/schema/domain-frontend` (return the content of json schema)

- [ ] When creating a new user, apply the default localization settings.

JIRA Issue: BA-111