feat: 用户软删除+30天自动清理，操作日志共享配方统一显示

用户删除改为软删除(deleted标记)，撤销时数据完整恢复（配方/收藏/库存等）。
30天后自动硬删除清理。操作日志中共享配方和采纳配方统一显示为"共享配方"。

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

backend/database.py

@@ -245,6 +245,12 @@ def init_db():
     if "en_name" not in cols:
         c.execute("ALTER TABLE recipes ADD COLUMN en_name TEXT DEFAULT ''")
 
+    # Migration: soft-delete for users
+    if "deleted" not in user_cols:
+        c.execute("ALTER TABLE users ADD COLUMN deleted INTEGER DEFAULT 0")
+    if "deleted_at" not in user_cols:
+        c.execute("ALTER TABLE users ADD COLUMN deleted_at TEXT")
+
     # Seed admin user if no users exist
     count = c.execute("SELECT COUNT(*) FROM users").fetchone()[0]
     if count == 0: