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/model.go

@@ -12,6 +12,14 @@ type User struct {
 	Dirs           []Dir
 	ApplyRule      bool
 	Rules          []Rule
+	WatchChats     []WatchChat
+}
+
+type WatchChat struct {
+	gorm.Model
+	UserID uint // User's database ID (not chat ID)
+	ChatID int64
+	Filter string
 }
 
 type Dir struct {