feat: implement watch for monitoring chat messages

- Added a new command handler for /watch that allows users to listen to messages from a specified chat and save them according to storage rules.
- Introduced filtering options for messages using regular expressions.
- Implemented functionality to start and stop watching chats, including error handling for invalid inputs and user settings.
- Created a new utility package for message element handling related to the watch feature.
- Updated the user model to manage watched chats, including methods to add, remove, and check if a chat is being watched.

database/chat.go

@@ -0,0 +1,39 @@
+package database
+
+import "context"
+
+func (user *User) WatchChat(ctx context.Context, chat WatchChat) error {
+	if user.WatchChats == nil {
+		user.WatchChats = make([]WatchChat, 0)
+	}
+
+	user.WatchChats = append(user.WatchChats, chat)
+	return db.WithContext(ctx).Save(user.WatchChats).Error
+}
+
+func (user *User) UnwatchChat(ctx context.Context, chatID int64) error {
+	var watchChat WatchChat
+	err := db.WithContext(ctx).Where("chat_id = ? AND user_id = ?", chatID, user.ID).First(&watchChat).Error
+	if err != nil {
+		return err
+	}
+	return db.WithContext(ctx).Unscoped().Delete(&watchChat).Error
+}
+
+func (user *User) WatchingChat(ctx context.Context, chatID int64) (bool, error) {
+	var count int64
+	err := db.WithContext(ctx).Model(&WatchChat{}).Where("chat_id = ? AND user_id = ?", chatID, user.ID).Count(&count).Error
+	if err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func GetWatchChatsByChatID(ctx context.Context, chatID int64) ([]*WatchChat, error) {
+	var watchChats []*WatchChat
+	err := db.WithContext(ctx).Where("chat_id = ?", chatID).Find(&watchChats).Error
+	if err != nil {
+		return nil, err
+	}
+	return watchChats, nil
+}