fix: 防御性编程确保数据库表存在，解决user_messages表不存在错误

- 添加_ensure_user_messages_table()方法：检查并创建user_messages表
- 添加_ensure_bot_messages_table()方法：检查并创建bot_messages表
- 添加_ensure_ocr_staging_table()方法：检查并创建ocr_staging表
- 在record_user_message、record_bot_reply、record_ocr_message、clear_ocr_staging方法开始处调用相应的防御性检查
- 解决0字节msg_history.db文件导致的'no such table: user_messages'错误

Task #1655

monitor.py

@@ -396,6 +396,9 @@ class MessageHistory:
 
     def record_bot_reply(self, platform: str, content: str):
         """记录bot回复到bot_messages表（环形缓冲区，max 10条/平台）"""
+        # ⭐ Task #1655: 确保表存在（防御性编程）
+        self._ensure_bot_messages_table()
+        
         content_hash = hashlib.md5(content.strip().encode('utf-8')).hexdigest()
         conn = self._get_conn()
         try:
@@ -539,8 +542,75 @@ class MessageHistory:
 
         return False
     
+    def _ensure_user_messages_table(self):
+        """确保user_messages表存在（防御性编程）"""
+        conn = self._get_conn()
+        try:
+            cursor = conn.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='user_messages'")
+            if not cursor.fetchone():
+                logger.warning("[msg-history] user_messages表不存在，重新创建")
+                conn.execute("""
+                    CREATE TABLE IF NOT EXISTS user_messages (
+                        id INTEGER PRIMARY KEY AUTOINCREMENT,
+                        platform TEXT NOT NULL,
+                        content TEXT NOT NULL,
+                        content_hash TEXT NOT NULL,
+                        created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+                    )
+                """)
+                conn.execute("CREATE INDEX IF NOT EXISTS idx_user_content_hash ON user_messages(content_hash)")
+                conn.execute("CREATE INDEX IF NOT EXISTS idx_user_platform_time ON user_messages(platform, created_at)")
+                conn.commit()
+        finally:
+            conn.close()
+
+    def _ensure_bot_messages_table(self):
+        """确保bot_messages表存在（防御性编程）"""
+        conn = self._get_conn()
+        try:
+            cursor = conn.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='bot_messages'")
+            if not cursor.fetchone():
+                logger.warning("[msg-history] bot_messages表不存在，重新创建")
+                conn.execute("""
+                    CREATE TABLE IF NOT EXISTS bot_messages (
+                        id INTEGER PRIMARY KEY AUTOINCREMENT,
+                        platform TEXT NOT NULL,
+                        content TEXT NOT NULL,
+                        content_hash TEXT NOT NULL,
+                        created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+                    )
+                """)
+                conn.execute("CREATE INDEX IF NOT EXISTS idx_bot_content_hash ON bot_messages(content_hash)")
+                conn.execute("CREATE INDEX IF NOT EXISTS idx_bot_platform_time ON bot_messages(platform, created_at)")
+                conn.commit()
+        finally:
+            conn.close()
+
+    def _ensure_ocr_staging_table(self):
+        """确保ocr_staging表存在（防御性编程）"""
+        conn = self._get_conn()
+        try:
+            cursor = conn.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='ocr_staging'")
+            if not cursor.fetchone():
+                logger.warning("[msg-history] ocr_staging表不存在，重新创建")
+                conn.execute("""
+                    CREATE TABLE IF NOT EXISTS ocr_staging (
+                        id INTEGER PRIMARY KEY AUTOINCREMENT,
+                        platform TEXT NOT NULL,
+                        content TEXT NOT NULL,
+                        content_hash TEXT NOT NULL,
+                        created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+                    )
+                """)
+                conn.commit()
+        finally:
+            conn.close()
+
     def record_user_message(self, platform: str, content: str):
         """记录用户消息到user_messages表（环形缓冲区，max 10条/平台）"""
+        # ⭐ Task #1655: 确保表存在（防御性编程）
+        self._ensure_user_messages_table()
+        
         # ⭐ Task #1654: 修复去重误判问题
         #   问题：exclude_self=False 导致自我比较产生100%相似度，误丢弃正常消息
         #   修复：使用 exclude_self=True 避免自我比较，只与历史消息比对
@@ -720,6 +790,9 @@ class MessageHistory:
     
     def record_ocr_message(self, platform: str, content: str):
         """记录OCR识别的消息到暂存表"""
+        # ⭐ Task #1655: 确保表存在（防御性编程）
+        self._ensure_ocr_staging_table()
+        
         content_hash = hashlib.md5(content.strip().encode('utf-8')).hexdigest()
         conn = self._get_conn()
         try:
@@ -735,6 +808,9 @@ class MessageHistory:
     
     def clear_ocr_staging(self):
         """清空OCR暂存表（重启时调用）"""
+        # ⭐ Task #1655: 确保表存在（防御性编程）
+        self._ensure_ocr_staging_table()
+        
         conn = self._get_conn()
         try:
             conn.execute("DELETE FROM ocr_staging")