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/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"
 )
@@ -33,6 +34,8 @@ type Params struct {
 	Database    *db.Database
 	Healthcheck *healthcheck.Healthcheck
 	Stats       *stats.Tracker
+	Broker      *broker.Broker
+	Service     *service.Service
 }
 
 const defaultIdleTimeout = 30 * 24 * time.Hour
@@ -48,6 +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
@@ -79,7 +83,8 @@ func New(
 		params:          &params,
 		log:             params.Logger.Get(),
 		hc:              params.Healthcheck,
-		broker:          broker.New(),
+		broker:          params.Broker,
+		svc:             params.Service,
 		hashcashVal:     hashcash.NewValidator(resource),
 		channelHashcash: hashcash.NewChannelValidator(),
 		loginLimiter:    ratelimit.New(loginRate, loginBurst),