fix: 油名迁移处理新旧名字同时存在的情况，避免UNIQUE冲突

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

backend/database.py

@@ -254,8 +254,13 @@ def init_db():
     # Migration: rename oils 西洋蓍草→西洋蓍草石榴籽, 元气→元气焕能
     _oil_renames = [("西洋蓍草", "西洋蓍草石榴籽"), ("元气", "元气焕能")]
     for old_name, new_name in _oil_renames:
-        exists = c.execute("SELECT 1 FROM oils WHERE name = ?", (old_name,)).fetchone()
-        if exists:
+        old_exists = c.execute("SELECT 1 FROM oils WHERE name = ?", (old_name,)).fetchone()
+        new_exists = c.execute("SELECT 1 FROM oils WHERE name = ?", (new_name,)).fetchone()
+        if old_exists and new_exists:
+            # Both exist: delete old, update recipe references to new
+            c.execute("DELETE FROM oils WHERE name = ?", (old_name,))
+            c.execute("UPDATE recipe_ingredients SET oil_name = ? WHERE oil_name = ?", (new_name, old_name))
+        elif old_exists:
             c.execute("UPDATE oils SET name = ? WHERE name = ?", (new_name, old_name))
             c.execute("UPDATE recipe_ingredients SET oil_name = ? WHERE oil_name = ?", (new_name, old_name))