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/seed.py

@@ -21,10 +21,13 @@ from .models import (
     MediaAsset,
     Product,
     ProductImage,
+    RouteSection,
+    RouteSectionProduct,
     SiteVersion,
     ThemeCard,
     utc_now,
 )
+from .route_sections import ROUTE_SECTION_DEFAULTS
 
 
 ROOT_DIR = Path(__file__).resolve().parent.parent
@@ -97,7 +100,7 @@ def load_products() -> list[dict]:
 
 def reset_guizhou_content(db: Session) -> dict:
     products = load_products()
-    for model in [SiteVersion, CampaignProduct, Campaign, ProductImage, Product, CtaBanner, ThemeCard, HeroSlide, DestinationAlias, Destination, MediaAsset]:
+    for model in [SiteVersion, CampaignProduct, RouteSectionProduct, Campaign, RouteSection, ProductImage, Product, CtaBanner, ThemeCard, HeroSlide, DestinationAlias, Destination, MediaAsset]:
         db.execute(delete(model))
     db.flush()
 
@@ -178,6 +181,18 @@ def reset_guizhou_content(db: Session) -> dict:
         for index, product in enumerate(linked_products):
             db.add(CampaignProduct(campaignId=campaign.id, productId=product.id, sortOrder=index))
 
+    for sort_order, (section_id, title, subtitle, start, end) in enumerate(ROUTE_SECTION_DEFAULTS):
+        section = RouteSection(id=section_id, title=title, subtitle=subtitle, sortOrder=sort_order)
+        db.add(section)
+        db.flush()
+        section_products = db.scalars(
+            select(Product)
+            .where(Product.sourceId >= start + 1, Product.sourceId <= end)
+            .order_by(Product.sourceId.asc())
+        ).all()
+        for index, product in enumerate(section_products):
+            db.add(RouteSectionProduct(sectionId=section.id, productId=product.id, sortOrder=index))
+
     snapshot = SiteVersion(
         title="guizhou-content-reset",
         status="published",
@@ -187,6 +202,7 @@ def reset_guizhou_content(db: Session) -> dict:
             "destinations": len(DESTINATIONS),
             "themeCards": len(THEMES),
             "products": len(products),
+            "routeSections": len(ROUTE_SECTION_DEFAULTS),
         },
     )
     db.add(snapshot)
@@ -197,6 +213,7 @@ def reset_guizhou_content(db: Session) -> dict:
         "themes": len(THEMES),
         "ctaBanners": len(CTAS),
         "products": len(products),
+        "routeSections": len(ROUTE_SECTION_DEFAULTS),
         "siteVersionId": snapshot.id,
     }