refactor: shared service layer, default IRC port, smaller functions

Wire up service.Service in HTTP handlers and delegate cleanupUser to
svc.BroadcastQuit for consistent quit/part logic across transports.
Default IRC_LISTEN_ADDR to :6667, remove unused import, fix all lint
issues (dogsled, funcorder, wrapcheck, varnamelen, nolintlint).

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

internal/ircserver/export_test.go

@@ -8,6 +8,7 @@ import (
 	"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/service"
 )
 
 // NewTestServer creates a Server suitable for testing.
@@ -18,11 +19,19 @@ func NewTestServer(
 	database *db.Database,
 	brk *broker.Broker,
 ) *Server {
+	svc := &service.Service{
+		DB:     database,
+		Broker: brk,
+		Config: cfg,
+		Log:    log,
+	}
+
 	return &Server{ //nolint:exhaustruct
 		log:      log,
 		cfg:      cfg,
 		database: database,
 		brk:      brk,
+		svc:      svc,
 		conns:    make(map[*Conn]struct{}),
 	}
 }