feat: add IRC wire protocol listener with shared service layer

Adds a backward-compatible IRC wire protocol listener (RFC 1459/2812)
with a shared service layer used by both IRC and HTTP transports.

- TCP listener on configurable port (default :6667)
- Full IRC protocol: NICK, USER, JOIN, PART, PRIVMSG, MODE, TOPIC, etc.
- Shared service layer (internal/service/) for consistent code paths
- Tier 2 join restrictions (ban, invite-only, key, limit) in service layer
- Ban check on PRIVMSG in service layer
- SetChannelFlag handles +i and +s modes
- Command dispatch via map[string]cmdHandler pattern
- EXPOSE 6667 in Dockerfile
- Service layer unit tests

closes #89

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,
 	}