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.

gateway/internal/config/config_test.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"testing"
+	"time"
 
 	pkgtel "scrabble/pkg/telemetry"
 )
@@ -45,3 +46,52 @@ func TestLoadMaxBodyBytes(t *testing.T) {
 		t.Fatal("Load: expected an error for a non-positive body cap, got nil")
 	}
 }
+
+// TestLoadAbuseDefaults verifies the anti-abuse ban defaults: disabled (prod-only),
+// the agreed thresholds, and no honeytoken.
+func TestLoadAbuseDefaults(t *testing.T) {
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	want := DefaultAbuse()
+	if c.Abuse != want {
+		t.Errorf("Abuse = %+v, want %+v", c.Abuse, want)
+	}
+	if c.Abuse.BanEnabled {
+		t.Error("ban must default to disabled (enabled only in prod)")
+	}
+}
+
+// TestLoadAbuseOverrides verifies the anti-abuse environment variables are parsed.
+func TestLoadAbuseOverrides(t *testing.T) {
+	t.Setenv("GATEWAY_ABUSE_BAN_ENABLED", "true")
+	t.Setenv("GATEWAY_ABUSE_BAN_THRESHOLD", "50")
+	t.Setenv("GATEWAY_ABUSE_BAN_WINDOW", "90s")
+	t.Setenv("GATEWAY_ABUSE_BAN_DURATION", "30m")
+	t.Setenv("GATEWAY_HONEYTOKEN", "deadbeef")
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	want := AbuseConfig{
+		BanEnabled:   true,
+		BanThreshold: 50,
+		BanWindow:    90 * time.Second,
+		BanDuration:  30 * time.Minute,
+		Honeytoken:   "deadbeef",
+	}
+	if c.Abuse != want {
+		t.Errorf("Abuse = %+v, want %+v", c.Abuse, want)
+	}
+}
+
+// TestLoadAbuseRejectsBadThreshold verifies an enabled ban with a non-positive
+// threshold fails validation.
+func TestLoadAbuseRejectsBadThreshold(t *testing.T) {
+	t.Setenv("GATEWAY_ABUSE_BAN_ENABLED", "true")
+	t.Setenv("GATEWAY_ABUSE_BAN_THRESHOLD", "0")
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: expected an error for an enabled ban with a zero threshold, got nil")
+	}
+}