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/routers/shared.py

@@ -1,16 +1,10 @@
 from sqlalchemy import select
 from sqlalchemy.orm import Session, selectinload
-from ..models import Campaign, CtaBanner, Destination, HeroSlide, MapImage, Product, ThemeCard
+from ..models import Campaign, CtaBanner, Destination, HeroSlide, MapImage, ThemeCard
+from ..route_sections import route_section_dict, route_section_query
 from ..serializers import destination_dict, model_dict
 
 
-ROUTE_SECTION_LABELS = [
-    ("routes", "经典人文打卡线路", 0, 8),
-    ("routes-outdoor", "极限山野户外野咖线路", 8, 16),
-    ("routes-mix", "人文+户外综合混搭线路", 16, 24),
-]
-
-
 def site_config(db: Session, active_only: bool, include_public_extras: bool = True) -> dict:
     hero_stmt = select(HeroSlide).order_by(HeroSlide.sortOrder.asc())
     destination_stmt = select(Destination).options(selectinload(Destination.aliases)).order_by(Destination.sortOrder.asc())
@@ -38,12 +32,9 @@ def site_config(db: Session, active_only: bool, include_public_extras: bool = Tr
     }
     if include_public_extras:
         campaigns = db.scalars(select(Campaign).where(Campaign.status == "published").order_by(Campaign.updatedAt.desc())).all()
-        products = db.scalars(
-            select(Product).where(Product.status == "published").order_by(Product.sortWeight.asc(), Product.createdAt.asc()).limit(48)
-        ).all()
+        route_sections = db.scalars(route_section_query()).all()
+        if active_only:
+            route_sections = [section for section in route_sections if section.isActive]
         result["campaigns"] = [model_dict(item) for item in campaigns]
-        result["routeSections"] = [
-            {"id": section_id, "title": title, "productIds": [item.id for item in products[start:end]]}
-            for section_id, title, start, end in ROUTE_SECTION_LABELS
-        ]
+        result["routeSections"] = [route_section_dict(item, public=True) for item in route_sections]
     return result