feat(gateway): temporary IP ban (fail2ban) fed by rejections + honeypot/honeytoken

Add a prod-only, in-memory IP ban enforced at the edge, fed by three signals:
sustained rate-limiter rejections (the IP-keyed public/email/admin classes — the
user class stays the backend soft-flag's concern), a honeypot decoy-path hit (the
contour caddy tags decoys with X-Scrabble-Honeypot and routes them to the gateway),
and a honeytoken (a planted bearer, GATEWAY_HONEYTOKEN). A banned IP is refused with
429 by the abuseGuard middleware before any work — covering the Connect edge, the
live stream and the static SPA/landing the per-op limiter never gated.

The ban is off by default: it keys by the real client IP the shared-NAT test contour
does not expose, so a ban there would be self-inflicted; detection still logs in the
contour, only the ban action is gated (GATEWAY_ABUSE_BAN_ENABLED). Rejection bans last
GATEWAY_ABUSE_BAN_DURATION; tripwire/honeytoken hits are near-zero-false-positive and
earn longer fixed bans. Each ban increments gateway_abuse_banned_total{reason}.

Operators see and lift active bans on the admin console's Throttled page; the gateway
syncs its active set to the backend every 30s (POST /api/v1/internal/bans/sync,
backend/internal/banview) and applies the operator unbans the response returns.

PRERELEASE phase AG. Docs baked into ARCHITECTURE / FUNCTIONAL (+ru) / both READMEs.

backend/internal/server/handlers.go

@@ -42,6 +42,11 @@ func (s *Server) registerRoutes() {
 		// admin console's throttled view and the high-rate auto-flag.
 		s.internal.POST("/ratelimit/report", s.handleRateLimitReport)
 	}
+	if s.banview != nil {
+		// The gateway's periodic active-ban sync: feeds the admin console's
+		// active-bans panel and returns the operator's pending unbans.
+		s.internal.POST("/bans/sync", s.handleBanSync)
+	}
 	u := s.user
 	if s.accounts != nil {
 		u.GET("/profile", s.handleProfile)