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/api/materials.py

@@ -265,3 +265,36 @@ async def is_collected(
         )
     )).scalars().all()
     return {"collected": len(rows) > 0, "count": len(rows)}
+
+
+@router.delete("/assets/{asset_id}")
+async def delete_asset(asset_id: str, db: AsyncSession = Depends(get_db)):
+    """删除一条素材记录（DB 行 + 本地文件；七牛等远程存储仅删引用）。"""
+    from services.storage import local_path
+
+    asset = await db.get(ProductAsset, UUID(asset_id))
+    if asset is None:
+        raise HTTPException(status_code=404, detail="素材不存在")
+
+    if asset.stored_url:
+        path = local_path(asset.stored_url)
+        if path is not None and path.is_file():
+            try:
+                path.unlink()
+            except OSError:  # noqa: BLE001
+                pass  # 文件删除失败不阻断记录删除
+
+    product_id = asset.product_id
+    group_key = asset.group_key
+    await db.delete(asset)
+
+    # 同步修正 asset_counts（generated 计数由生图回写时累加）
+    product = await db.get(Product, product_id)
+    if product is not None:
+        counts = dict(product.asset_counts or {})
+        if group_key in counts:
+            counts[group_key] = max(0, int(counts.get(group_key) or 0) - 1)
+            product.asset_counts = counts
+
+    await db.commit()
+    return {"deleted": True}