fix: 修复邮箱绑定验证错误及多项改进

1. 修复email_verified字段缺失导致的500错误
2. 将邮件主题从"知识管理平台"改为"自动化学习"
3. 增大验证码字体（28->42）和图片尺寸（120x40->160x60）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database.py

@@ -834,6 +834,13 @@ def update_user_email(user_id, email, verified=False):
     """更新用户邮箱"""
     with db_pool.get_db() as conn:
         cursor = conn.cursor()
+        # 先检查email_verified字段是否存在，不存在则添加
+        try:
+            cursor.execute('SELECT email_verified FROM users LIMIT 1')
+        except:
+            cursor.execute('ALTER TABLE users ADD COLUMN email_verified INTEGER DEFAULT 0')
+            conn.commit()
+
         cursor.execute('''
             UPDATE users
             SET email = ?, email_verified = ?
@@ -843,6 +850,41 @@ def update_user_email(user_id, email, verified=False):
         return cursor.rowcount > 0
 
 
+def update_user_email_notify(user_id, enabled):
+    """更新用户邮件通知偏好"""
+    with db_pool.get_db() as conn:
+        cursor = conn.cursor()
+        # 先检查字段是否存在
+        try:
+            cursor.execute('SELECT email_notify_enabled FROM users LIMIT 1')
+        except:
+            cursor.execute('ALTER TABLE users ADD COLUMN email_notify_enabled INTEGER DEFAULT 1')
+            conn.commit()
+
+        cursor.execute('''
+            UPDATE users
+            SET email_notify_enabled = ?
+            WHERE id = ?
+        ''', (int(enabled), user_id))
+        conn.commit()
+        return cursor.rowcount > 0
+
+
+def get_user_email_notify(user_id):
+    """获取用户邮件通知偏好（默认开启）"""
+    with db_pool.get_db() as conn:
+        cursor = conn.cursor()
+        # 先检查字段是否存在
+        try:
+            cursor.execute('SELECT email_notify_enabled FROM users WHERE id = ?', (user_id,))
+            row = cursor.fetchone()
+            if row is None:
+                return True
+            return bool(row[0]) if row[0] is not None else True
+        except:
+            return True  # 字段不存在时默认开启
+
+
 def get_all_users():
     """获取所有用户"""
     with db_pool.get_db() as conn: