feat: 购油方案 — 会员请求+老师定制+购油清单

后端:
- oil_plans/oil_plan_recipes 表
- 7个API: teachers列表、方案CRUD、配方增删、购油清单计算
- 购油清单自动算月消耗、瓶数、费用，交叉比对库存

前端:
- 会员端(库存页): 请求方案→选老师→描述需求；查看购油清单
- 老师端(用户管理): 接收请求→选配方+频率→激活方案
- plans store 管理状态

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

backend/database.py

@@ -245,6 +245,24 @@ 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: