refactor: rename MessageType to IRCMessageType with full API

- Rename MessageType to IRCMessageType (custom int type)
- .Name() returns just the name (e.g. "RPL_LUSEROP")
- .String() returns name + code (e.g. "RPL_LUSEROP <252>")
- .Code() returns zero-padded 3-digit string (e.g. "252")
- .Int() returns the bare int value
- Add FromInt() package-level function with validation
- Update all call sites to use new type and methods

internal/handlers/api.go

@@ -398,12 +398,12 @@ func (hdlr *Handlers) serverName() string {
 func (hdlr *Handlers) enqueueNumeric(
 	ctx context.Context,
 	clientID int64,
-	code irc.MessageType,
+	code irc.IRCMessageType,
 	nick string,
 	params []string,
 	text string,
 ) {
-	command := code.String()
+	command := code.Code()
 
 	body, err := json.Marshal([]string{text})
 	if err != nil {
@@ -969,7 +969,7 @@ func (hdlr *Handlers) respondIRCError(
 	writer http.ResponseWriter,
 	request *http.Request,
 	clientID, sessionID int64,
-	code irc.MessageType,
+	code irc.IRCMessageType,
 	nick string,
 	params []string,
 	text string,