Domain-level frontend settings

[achimnol]

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.