revert: 删除购油方案功能，修复MyDiary模板错误

完全移除oil_plans相关代码：
- 后端: 7个API端点、2个数据库表
- 前端: plans store、Inventory方案UI、UserManagement方案编辑器
- UserMenu: 方案通知按钮
- 修复MyDiary.vue多余的section-card div导致的构建失败

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

backend/database.py

@@ -247,24 +247,6 @@ def init_db():
     if "en_name" not in cols:
         c.execute("ALTER TABLE recipes ADD COLUMN en_name TEXT DEFAULT ''")
 
-    # Oil plans
-    c.execute("""CREATE TABLE IF NOT EXISTS oil_plans (
-        id INTEGER PRIMARY KEY AUTOINCREMENT,
-        user_id INTEGER NOT NULL,
-        teacher_id INTEGER,
-        title TEXT DEFAULT '',
-        health_desc TEXT DEFAULT '',
-        status TEXT NOT NULL DEFAULT 'pending',
-        created_at TEXT DEFAULT (datetime('now'))
-    )""")
-    c.execute("""CREATE TABLE IF NOT EXISTS oil_plan_recipes (
-        id INTEGER PRIMARY KEY AUTOINCREMENT,
-        plan_id INTEGER NOT NULL REFERENCES oil_plans(id) ON DELETE CASCADE,
-        recipe_name TEXT NOT NULL,
-        ingredients TEXT NOT NULL DEFAULT '[]',
-        times_per_month INTEGER NOT NULL DEFAULT 1
-    )""")
-
     # Seed admin user if no users exist
     count = c.execute("SELECT COUNT(*) FROM users").fetchone()[0]
     if count == 0: