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/db/queries.go

@@ -746,8 +746,8 @@ func scanMessages(
 			code, _ := strconv.Atoi(msg.Command)
 			msg.Code = code
 
-			if name := irc.Name(irc.MessageType(code)); name != "" {
-				msg.Command = name
+			if mt, err := irc.FromInt(code); err == nil {
+				msg.Command = mt.Name()
 			}
 		}