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/handlers/handlers.go

@@ -17,6 +17,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/healthcheck"
 	"git.eeqj.de/sneak/neoirc/internal/logger"
 	"git.eeqj.de/sneak/neoirc/internal/ratelimit"
+	"git.eeqj.de/sneak/neoirc/internal/service"
 	"git.eeqj.de/sneak/neoirc/internal/stats"
 	"go.uber.org/fx"
 )
@@ -34,6 +35,7 @@ type Params struct {
 	Healthcheck *healthcheck.Healthcheck
 	Stats       *stats.Tracker
 	Broker      *broker.Broker
+	Service     *service.Service
 }
 
 const defaultIdleTimeout = 30 * 24 * time.Hour
@@ -49,6 +51,7 @@ type Handlers struct {
 	log             *slog.Logger
 	hc              *healthcheck.Healthcheck
 	broker          *broker.Broker
+	svc             *service.Service
 	hashcashVal     *hashcash.Validator
 	channelHashcash *hashcash.ChannelValidator
 	loginLimiter    *ratelimit.Limiter
@@ -81,6 +84,7 @@ func New(
 		log:             params.Logger.Get(),
 		hc:              params.Healthcheck,
 		broker:          params.Broker,
+		svc:             params.Service,
 		hashcashVal:     hashcash.NewValidator(resource),
 		channelHashcash: hashcash.NewChannelValidator(),
 		loginLimiter:    ratelimit.New(loginRate, loginBurst),