feat: add IRC wire protocol listener with shared service layer

Add a traditional IRC wire protocol listener (RFC 1459/2812) on
configurable port (default :6667), sharing business logic with
the HTTP API via a new service layer.

- IRC listener: NICK, USER, PASS, JOIN, PART, PRIVMSG, NOTICE,
  TOPIC, MODE, KICK, QUIT, NAMES, LIST, WHOIS, WHO, AWAY, OPER,
  INVITE, LUSERS, MOTD, PING/PONG, CAP
- Service layer: shared logic for both transports including
  channel join (with Tier 2 checks: ban/invite/key/limit),
  message send (with ban + moderation checks), nick change,
  topic, kick, mode, quit broadcast, away, oper, invite
- BroadcastQuit uses FanOut pattern (one insert, N enqueues)
- HTTP handlers delegate to service for all command logic
- Tier 2 mode operations (+b/+i/+s/+k/+l) use service methods

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -50,6 +50,7 @@ type Config struct {
 	OperPassword       string
 	LoginRateLimit     float64
 	LoginRateBurst     int
+	IRCListenAddr      string
 	params             *Params
 	log                *slog.Logger
 }
@@ -86,6 +87,7 @@ func New(
 	viper.SetDefault("NEOIRC_OPER_PASSWORD", "")
 	viper.SetDefault("LOGIN_RATE_LIMIT", "1")
 	viper.SetDefault("LOGIN_RATE_BURST", "5")
+	viper.SetDefault("IRC_LISTEN_ADDR", ":6667")
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -116,6 +118,7 @@ func New(
 		OperPassword:       viper.GetString("NEOIRC_OPER_PASSWORD"),
 		LoginRateLimit:     viper.GetFloat64("LOGIN_RATE_LIMIT"),
 		LoginRateBurst:     viper.GetInt("LOGIN_RATE_BURST"),
+		IRCListenAddr:      viper.GetString("IRC_LISTEN_ADDR"),
 		log:                log,
 		params:             &params,
 	}