feat: add OPER command and oper-only WHOIS client info

- Add OPER command with NEOIRC_OPER_NAME/NEOIRC_OPER_PASSWORD config
- Add is_oper column to sessions table
- Add RPL_WHOISACTUALLY (338): show client IP/hostname to opers
- Add RPL_WHOISOPERATOR (313): show oper status in WHOIS
- Add GetOperCount for accurate LUSERS oper count
- Fix README schema: add ip/is_oper to sessions, ip/hostname to clients
- Add OPER command documentation and numeric references to README
- Refactor executeWhois to stay under funlen limit
- Add comprehensive tests for OPER auth, oper WHOIS, non-oper WHOIS

Closes #81

pkg/irc/commands.go

@@ -11,6 +11,7 @@ const (
 	CmdNames   = "NAMES"
 	CmdNick    = "NICK"
 	CmdNotice  = "NOTICE"
+	CmdOper    = "OPER"
 	CmdPart    = "PART"
 	CmdPing    = "PING"
 	CmdPong    = "PONG"