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/route_sections.py

@@ -0,0 +1,91 @@
+from fastapi import HTTPException
+from sqlalchemy import delete, select
+from sqlalchemy.orm import Session, selectinload
+
+from .models import Product, RouteSection, RouteSectionProduct
+from .serializers import model_dict
+
+
+ROUTE_SECTION_DEFAULTS = [
+    ("routes", "经典人文打卡线路", "黄果树、荔波小七孔、千户苗寨、镇远古城、梵净山一次串联", 0, 8),
+    ("routes-outdoor", "极限山野户外野咖线路", "溶洞、峡谷、漂流、峰林骑行和山野咖啡组合", 8, 16),
+    ("routes-mix", "人文+户外综合混搭线路", "非遗村寨、古城夜游、自然轻探险和精品住宿同程安排", 16, 24),
+]
+
+
+def route_section_error(status_code: int, message: str, code: str, details: dict | None = None) -> None:
+    raise HTTPException(status_code=status_code, detail={"message": message, "code": code, "details": details or {}})
+
+
+def route_section_dict(item: RouteSection, *, public: bool = False) -> dict:
+    links = sorted(item.products or [], key=lambda link: (link.sortOrder, link.productId))
+    if public:
+        links = [link for link in links if link.product and link.product.status == "published"]
+
+    data = {
+        "id": item.id,
+        "title": item.title,
+        "subtitle": item.subtitle,
+        "productIds": [link.productId for link in links],
+        "isActive": item.isActive,
+    }
+    if not public:
+        base = model_dict(item)
+        data.update(
+            {
+                "sortOrder": item.sortOrder,
+                "createdAt": base["createdAt"],
+                "updatedAt": base["updatedAt"],
+            }
+        )
+    return data
+
+
+def route_section_query():
+    return (
+        select(RouteSection)
+        .options(selectinload(RouteSection.products).selectinload(RouteSectionProduct.product))
+        .order_by(RouteSection.sortOrder.asc())
+    )
+
+
+
+
+def validate_route_section_product_ids(db: Session, section_id: str, product_ids: list[str]) -> None:
+    if len(set(product_ids)) != len(product_ids):
+        route_section_error(422, "线路商品不能重复", "ROUTE_SECTION_PRODUCT_DUPLICATE", {"sectionId": section_id})
+    if not product_ids:
+        return
+
+    products = db.scalars(select(Product).where(Product.id.in_(product_ids))).all()
+    found_ids = {product.id for product in products}
+    missing_ids = [product_id for product_id in product_ids if product_id not in found_ids]
+    if missing_ids:
+        route_section_error(422, "线路商品不存在", "ROUTE_SECTION_PRODUCT_NOT_FOUND", {"productIds": missing_ids})
+
+    conflicts = db.scalars(
+        select(RouteSectionProduct).where(
+            RouteSectionProduct.productId.in_(product_ids),
+            RouteSectionProduct.sectionId != section_id,
+        )
+    ).all()
+    if conflicts:
+        conflict = conflicts[0]
+        route_section_error(
+            409,
+            "同一线路商品不能同时属于多个精选线路子分组",
+            "ROUTE_SECTION_PRODUCT_CONFLICT",
+            {"productId": conflict.productId, "sectionId": conflict.sectionId},
+        )
+
+
+def replace_route_section_products(db: Session, section: RouteSection, product_ids: list[str]) -> None:
+    validate_route_section_product_ids(db, section.id, product_ids)
+    db.execute(delete(RouteSectionProduct).where(RouteSectionProduct.sectionId == section.id))
+    db.flush()
+    section.products = []
+    for index, product_id in enumerate(product_ids):
+        link = RouteSectionProduct(sectionId=section.id, productId=product_id, sortOrder=index)
+        section.products.append(link)
+        db.add(link)
+