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/db/queries.go

@@ -746,7 +746,7 @@ func scanMessages(
 			code, _ := strconv.Atoi(msg.Command)
 			msg.Code = code
 
-			if name := irc.Name(code); name != "" {
+			if name := irc.Name(irc.MessageType(code)); name != "" {
 				msg.Command = name
 			}
 		}