Enforce channel keys, invites, +s and +H on the IRC listener (closes #122)

The IRC listener's JOIN now passes the channel key on, INVITE records
the invite, LIST and WHOIS hide +s channels from non-members with the
queries the HTTP API uses, and PRIVMSG to a +H channel gets 404.

The INVITE checks and the channel hashcash check move from the HTTP
handlers into the service, so both transports run the same code. An IRC
client cannot attach a stamp, so its PRIVMSG to a +H channel always
fails that check. The unfiltered channel list query, now unused, is
removed.

Model: opus-5-5

internal/handlers/handlers.go

@@ -55,16 +55,15 @@ const spentHashcashTTL = 365 * 24 * time.Hour
 
 // Handlers manages HTTP request handling.
 type Handlers struct {
-	params          *Params
-	log             *slog.Logger
-	hc              *healthcheck.Healthcheck
-	broker          *broker.Broker
-	svc             *service.Service
-	hashcashVal     *hashcash.Validator
-	channelHashcash *hashcash.ChannelValidator
-	loginLimiter    *ratelimit.Limiter
-	stats           *stats.Tracker
-	cancelCleanup   context.CancelFunc
+	params        *Params
+	log           *slog.Logger
+	hc            *healthcheck.Healthcheck
+	broker        *broker.Broker
+	svc           *service.Service
+	hashcashVal   *hashcash.Validator
+	loginLimiter  *ratelimit.Limiter
+	stats         *stats.Tracker
+	cancelCleanup context.CancelFunc
 }
 
 // New creates a new Handlers instance.
@@ -88,15 +87,14 @@ func New(
 	}
 
 	hdlr := &Handlers{ //nolint:exhaustruct // cancelCleanup set in startCleanup
-		params:          &params,
-		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),
-		stats:           params.Stats,
+		params:       &params,
+		log:          params.Logger.Get(),
+		hc:           params.Healthcheck,
+		broker:       params.Broker,
+		svc:          params.Service,
+		hashcashVal:  hashcash.NewValidator(resource),
+		loginLimiter: ratelimit.New(loginRate, loginBurst),
+		stats:        params.Stats,
 	}
 
 	lifecycle.Append(fx.Hook{