refactor(server): 移除已放弃的 Ozon API 直传模块，新增素材删除接口

- 删除 shops/publish/categories/ozon 相关路由、模型、schema 与客户端服务（V2.1 决策放弃 API 直传，改人工上传）
- materials 新增 DELETE /assets/{id}：删除素材记录与本地文件，并同步修正 asset_counts
- 试算页支持单张素材删除，生图弹窗交互微调
- 新增 docs/v2.1/HANDOFF.md 工作交接说明，README 补充索引

server/services/suite_service.py

@@ -46,10 +46,16 @@ def validate_model(provider_name: str, model: str | None) -> None:
         raise ValueError(f"不支持的模型: {model}（rightapi 支持: {RIGHTAPI_MODELS}）")
 
 
-async def append_generated_asset(product_id: str, image: TaskImage) -> str | None:
+async def append_generated_asset(
+    product_id: str,
+    image: TaskImage,
+    after_asset_id: str | None = None,
+) -> str | None:
     """把一张生成完成的图追加为 product_assets(generated)，并累加 asset_counts。返回 asset_id。
 
-    作为 run_suite 的逐张回调使用（签名须为 (image)），调用方用 partial 绑定 product_id。
+    after_asset_id：插入锚点——新素材排在锚点之后（其余素材顺次后移），方便与原图对比；
+    缺省/锚点无效时追加到组尾。作为 run_suite 的逐张回调使用时签名须为 (image)，
+    调用方用 partial 绑定 product_id。
     """
     from sqlalchemy import func, select
 
@@ -58,17 +64,41 @@ async def append_generated_asset(product_id: str, image: TaskImage) -> str | Non
 
     pid = uuid.UUID(product_id)
     async with get_session_factory()() as db:
-        count = await db.scalar(
-            select(func.count(ProductAsset.id)).where(
-                ProductAsset.product_id == pid,
-                ProductAsset.group_key == "generated",
+        sort_order: int | None = None
+        if after_asset_id:
+            try:
+                anchor = await db.get(ProductAsset, UUID(after_asset_id))
+            except ValueError:
+                anchor = None
+            if anchor is not None and anchor.product_id == pid and anchor.group_key == "generated":
+                # 锚点之后的素材顺次后移，腾出插入位
+                followers = (
+                    await db.scalars(
+                        select(ProductAsset).where(
+                            ProductAsset.product_id == pid,
+                            ProductAsset.group_key == "generated",
+                            ProductAsset.sort_order > anchor.sort_order,
+                        )
+                    )
+                ).scalars().all()
+                for follower in followers:
+                    follower.sort_order += 1
+                sort_order = anchor.sort_order + 1
+
+        if sort_order is None:
+            sort_order = await db.scalar(
+                select(func.coalesce(func.max(ProductAsset.sort_order), -1)).where(
+                    ProductAsset.product_id == pid,
+                    ProductAsset.group_key == "generated",
+                )
             )
-        )
+            sort_order = (sort_order or 0) + 1
+
         asset = ProductAsset(
             product_id=pid,
             group_key="generated",
             variant_name=None,
-            sort_order=count or 0,
+            sort_order=sort_order,
             type="img",
             source_url="",
             stored_url=image.url,