Replace hardcoded homepage featured route groups with a fully managed dynamic system:
- add database models RouteSection and RouteSectionProduct for storing route groups and their associated products
- create Alembic migration 0004_route_sections for the new tables
- extend SiteConfigPatchIn schema with subtitle and productIds fields
- refactor shared site_config utility to load dynamic route sections instead of fixed groups
- implement admin CRUD API with validation for duplicate/conflicting product associations
- update public and admin API documentation to reflect the new system
- add default route section seed data and comprehensive test coverage
