refactor: add irc.MessageType custom type with Name() and String() methods

- Create 'type MessageType int' in pkg/irc/numerics.go
- Type all Rpl*/Err* numeric constants as MessageType
- Add Name() method returning the standard IRC name (e.g. RPL_WELCOME)
- Add String() method returning zero-padded numeric (e.g. 001)
- Update enqueueNumeric/respondIRCError to accept MessageType
- Update internal/db to use MessageType conversion for Name() calls
- Change names map key type from int to MessageType

Refs #52

internal/handlers/api.go

@@ -398,12 +398,12 @@ func (hdlr *Handlers) serverName() string {
 func (hdlr *Handlers) enqueueNumeric(
 	ctx context.Context,
 	clientID int64,
-	code int,
+	code irc.MessageType,
 	nick string,
 	params []string,
 	text string,
 ) {
-	command := fmt.Sprintf("%03d", code)
+	command := code.String()
 
 	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 int,
+	code irc.MessageType,
 	nick string,
 	params []string,
 	text string,