List the user modes in the 004 reply (closes #220)

Both transports sent an empty string as the user modes in RPL_MYINFO.
On the IRC listener that left an empty parameter in the middle of the
line, so a client that splits on spaces read the channel modes as the
user modes. The reply now lists o and w, the user modes the server
supports. Both transports read them from one constant in
internal/service, next to the code that decides which user modes MODE
accepts. The README and the schema notes now show what the HTTP API
sends.

Model: opus-5-5

internal/service/service.go

@@ -1169,6 +1169,12 @@ func (s *Service) ApplyUserMode(
 	return s.QueryUserMode(ctx, sessionID)
 }
 
+// UserModes lists the user modes the server supports, as
+// RPL_MYINFO sends them: o (server operator) and w
+// (receives WALLOPS). parseUserModeString must accept the
+// same letters.
+const UserModes = "ow"
+
 // parseUserModeString parses a user mode string such as
 // "+w", "-o" or "+w-o" into the new value of each flag it
 // names, or nil for a flag it does not name; a later letter