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/ircserver/server.go

@@ -0,0 +1,157 @@
+package ircserver
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"net"
+	"sync"
+
+	"git.eeqj.de/sneak/neoirc/internal/broker"
+	"git.eeqj.de/sneak/neoirc/internal/config"
+	"git.eeqj.de/sneak/neoirc/internal/db"
+	"git.eeqj.de/sneak/neoirc/internal/logger"
+	"git.eeqj.de/sneak/neoirc/internal/service"
+	"go.uber.org/fx"
+)
+
+// Params defines the dependencies for creating an IRC
+// Server.
+type Params struct {
+	fx.In
+
+	Logger   *logger.Logger
+	Config   *config.Config
+	Database *db.Database
+	Broker   *broker.Broker
+	Service  *service.Service
+}
+
+// Server is the TCP IRC protocol server.
+type Server struct {
+	log      *slog.Logger
+	cfg      *config.Config
+	database *db.Database
+	brk      *broker.Broker
+	svc      *service.Service
+	listener net.Listener
+	mu       sync.Mutex
+	conns    map[*Conn]struct{}
+	cancel   context.CancelFunc
+}
+
+// New creates a new IRC Server and registers its lifecycle
+// hooks.  The listener is only started if IRC_LISTEN_ADDR
+// is configured; otherwise the server is inert.
+func New(
+	lifecycle fx.Lifecycle,
+	params Params,
+) *Server {
+	srv := &Server{
+		log:      params.Logger.Get(),
+		cfg:      params.Config,
+		database: params.Database,
+		brk:      params.Broker,
+		svc:      params.Service,
+		conns:    make(map[*Conn]struct{}),
+		listener: nil,
+		cancel:   nil,
+		mu:       sync.Mutex{},
+	}
+
+	listenAddr := params.Config.IRCListenAddr
+	if listenAddr == "" {
+		return srv
+	}
+
+	lifecycle.Append(fx.Hook{
+		OnStart: func(ctx context.Context) error {
+			return srv.start(ctx, listenAddr)
+		},
+		OnStop: func(_ context.Context) error {
+			srv.stop()
+
+			return nil
+		},
+	})
+
+	return srv
+}
+
+// start begins listening for TCP connections.
+//
+//nolint:contextcheck // long-lived server ctx, not the short Fx one
+func (s *Server) start(_ context.Context, addr string) error {
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		return fmt.Errorf("irc listen: %w", err)
+	}
+
+	s.listener = ln
+
+	ctx, cancel := context.WithCancel(context.Background())
+	s.cancel = cancel
+
+	s.log.Info(
+		"irc server listening", "addr", addr,
+	)
+
+	go s.acceptLoop(ctx)
+
+	return nil
+}
+
+// stop shuts down the listener and all connections.
+func (s *Server) stop() {
+	if s.cancel != nil {
+		s.cancel()
+	}
+
+	if s.listener != nil {
+		s.listener.Close() //nolint:errcheck,gosec
+	}
+
+	s.mu.Lock()
+	for c := range s.conns {
+		c.conn.Close() //nolint:errcheck,gosec
+	}
+	s.mu.Unlock()
+}
+
+// acceptLoop accepts new connections.
+func (s *Server) acceptLoop(ctx context.Context) {
+	for {
+		tcpConn, err := s.listener.Accept()
+		if err != nil {
+			select {
+			case <-ctx.Done():
+				return
+			default:
+				s.log.Error(
+					"irc accept error", "error", err,
+				)
+
+				continue
+			}
+		}
+
+		client := newConn(
+			ctx, tcpConn, s.log,
+			s.database, s.brk, s.cfg, s.svc,
+		)
+
+		s.mu.Lock()
+		s.conns[client] = struct{}{}
+		s.mu.Unlock()
+
+		go func() {
+			defer func() {
+				s.mu.Lock()
+				delete(s.conns, client)
+				s.mu.Unlock()
+			}()
+
+			client.serve(ctx)
+		}()
+	}
+}