feat: 配方volume字段存储编辑器选择的容量

- 后端: recipes表新增volume列，API返回/保存volume
- 前端: 保存时发送formVolume，编辑时优先用stored volume
- 容量显示优先级: stored volume > 椰子油计算 > 产品ml求和
- 修复编辑器容量选择保存后不生效的bug

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/database.py

@@ -248,6 +248,8 @@ def init_db():
         c.execute("ALTER TABLE recipes ADD COLUMN updated_by INTEGER")
     if "en_name" not in cols:
         c.execute("ALTER TABLE recipes ADD COLUMN en_name TEXT DEFAULT ''")
+    if "volume" not in cols:
+        c.execute("ALTER TABLE recipes ADD COLUMN volume TEXT DEFAULT ''")
 
     # Seed admin user if no users exist
     count = c.execute("SELECT COUNT(*) FROM users").fetchone()[0]