feat(route-sections): add dynamic route sections

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

app/schemas.py

@@ -121,6 +121,7 @@ class LeadQuery(BaseModel):
 class SiteConfigPatchIn(BaseModel):
     title: str | None = None
     kicker: str | None = None
+    subtitle: str | None = None
     name: str | None = None
     slug: str | None = None
     region: str | None = None
@@ -132,6 +133,7 @@ class SiteConfigPatchIn(BaseModel):
     priceAmount: int | None = Field(default=None, ge=0)
     priceUnit: str | None = None
     tags: list[str] | None = None
+    productIds: list[str] | None = None
     status: str | None = None
     startsAt: datetime | None = None
     endsAt: datetime | None = None