feat: implement Tier 1 IRC numerics

- AWAY command with RPL_AWAY (301), RPL_UNAWAY (305), RPL_NOWAWAY (306)
- RPL_WHOISIDLE (317) with idle time and signon time in WHOIS
- RPL_TOPICWHOTIME (333) with topic setter and timestamp
- Schema migration 002 for away_message and topic metadata columns
- Refactor deliverJoinNumerics into smaller helper functions

internal/irc/commands.go

@@ -2,6 +2,7 @@ package irc
 
 // IRC command names (RFC 1459 / RFC 2812).
 const (
+	CmdAway    = "AWAY"
 	CmdJoin    = "JOIN"
 	CmdList    = "LIST"
 	CmdLusers  = "LUSERS"