fix: 软删除字段NULL兼容，用COALESCE避免已有用户无法登录

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

backend/database.py

@@ -248,6 +248,7 @@ def init_db():
     # Migration: soft-delete for users
     if "deleted" not in user_cols:
         c.execute("ALTER TABLE users ADD COLUMN deleted INTEGER DEFAULT 0")
+        c.execute("UPDATE users SET deleted = 0 WHERE deleted IS NULL")
     if "deleted_at" not in user_cols:
         c.execute("ALTER TABLE users ADD COLUMN deleted_at TEXT")