feat: add traditional IRC wire protocol listener on configurable port

Add a backward-compatible IRC protocol listener (RFC 1459/2812) that
allows standard IRC clients (irssi, weechat, hexchat, etc.) to connect
directly via TCP.

Key features:
- TCP listener on configurable port (IRC_LISTEN_ADDR env var, e.g. :6667)
- Full IRC wire protocol parsing and formatting
- Connection registration (NICK + USER + optional PASS)
- Channel operations: JOIN, PART, MODE, TOPIC, NAMES, LIST, KICK, INVITE
- Messaging: PRIVMSG, NOTICE (channel and direct)
- Info commands: WHO, WHOIS, LUSERS, MOTD, AWAY
- Operator support: OPER (with configured credentials)
- PING/PONG keepalive
- CAP negotiation (for modern client compatibility)
- Full bridge to HTTP/JSON API (shared DB, broker, sessions)
- Real-time message relay via broker notifications
- Comprehensive test suite (parser + integration tests)

The IRC listener is an optional component — disabled when IRC_LISTEN_ADDR
is empty (the default). The Broker is now an Fx-provided dependency shared
between HTTP handlers and the IRC server.

closes #89

internal/ircserver/export_test.go

@@ -0,0 +1,43 @@
+package ircserver
+
+import (
+	"context"
+	"log/slog"
+	"net"
+
+	"git.eeqj.de/sneak/neoirc/internal/broker"
+	"git.eeqj.de/sneak/neoirc/internal/config"
+	"git.eeqj.de/sneak/neoirc/internal/db"
+)
+
+// NewTestServer creates a Server suitable for testing.
+// The caller must call Stop() when finished.
+func NewTestServer(
+	log *slog.Logger,
+	cfg *config.Config,
+	database *db.Database,
+	brk *broker.Broker,
+) *Server {
+	return &Server{ //nolint:exhaustruct
+		log:      log,
+		cfg:      cfg,
+		database: database,
+		brk:      brk,
+		conns:    make(map[*Conn]struct{}),
+	}
+}
+
+// Start exposes the unexported start method for tests.
+func (s *Server) Start(addr string) error {
+	return s.start(context.Background(), addr)
+}
+
+// Stop exposes the unexported stop method for tests.
+func (s *Server) Stop() {
+	s.stop()
+}
+
+// Listener returns the server's net.Listener for tests.
+func (s *Server) Listener() net.Listener {
+	return s.listener
+}