feat(edge): community IP blocklist (Spamhaus DROP) at the gateway

Refuse a client whose IP is in a curated CIDR feed with 403 in the same
abuseGuard, before the fail2ban ban. Prod-only (keys by real client IP), off by
default.

- ratelimit.Blocklist: a sorted-range IPv4 matcher (binary search) with an
  allowlist checked first; ParseDROP reads the feed; ApplyRefresh keeps the
  last-good feed on a transient fetch failure and drops it fail-open once stale
  (better to under-block than block a legitimate client on a frozen feed). A
  separate static CIDR set, not the per-IP fail2ban store.
- gateway: a refresher goroutine re-fetches every few hours (bounded fetch + size
  cap); config GATEWAY_BLOCKLIST_{ENABLED,URL,ALLOW,REFRESH,MAX_STALENESS}.
  IPv6 is not matched (a v6 client is still covered by fail2ban / the honeypot).
- observability: gateway_blocklist_blocked_total + entries/age gauges; a Grafana
  alert warns before the feed is dropped; service-overview panels.
- deploy: compose env + write-prod-env.sh + prod-deploy/rollback wiring (opt-in
  via PROD_ vars).
- docs (ARCHITECTURE, deploy/README); unit tests (match, allowlist, IPv6 skip,
  parser, fault-tolerance) + a 403 abuse-guard integration test.

gateway/internal/ratelimit/blocklist_test.go

@@ -0,0 +1,106 @@
+package ratelimit
+
+import (
+	"errors"
+	"net"
+	"strings"
+	"testing"
+	"time"
+)
+
+// cidrs parses CIDR strings into networks for a test fixture.
+func cidrs(t *testing.T, ss ...string) []*net.IPNet {
+	t.Helper()
+	out := make([]*net.IPNet, 0, len(ss))
+	for _, s := range ss {
+		_, n, err := net.ParseCIDR(s)
+		if err != nil {
+			t.Fatalf("bad cidr %q: %v", s, err)
+		}
+		out = append(out, n)
+	}
+	return out
+}
+
+func TestBlocklistBlocked(t *testing.T) {
+	bl := NewBlocklist(true, cidrs(t, "10.20.30.0/24")) // allowlist
+	bl.SetCIDRs(cidrs(t, "1.2.3.0/24", "203.0.113.5/32", "198.51.100.0/28", "10.20.30.0/24"), time.Now())
+	cases := map[string]bool{
+		"1.2.3.0":       true,  // range start
+		"1.2.3.255":     true,  // range end
+		"1.2.4.0":       false, // just past the /24
+		"203.0.113.5":   true,  // /32 exact
+		"203.0.113.6":   false,
+		"198.51.100.15": true,  // within /28
+		"198.51.100.16": false, // past the /28
+		"9.9.9.9":       false, // not listed
+		"10.20.30.99":   false, // listed BUT allowlisted — never blocked
+		"::1":           false, // IPv6 — never matched here
+		"not-an-ip":     false,
+	}
+	for ip, want := range cases {
+		if got := bl.Blocked(ip); got != want {
+			t.Errorf("Blocked(%q) = %v, want %v", ip, got, want)
+		}
+	}
+}
+
+func TestBlocklistDisabledOrEmpty(t *testing.T) {
+	off := NewBlocklist(false, nil)
+	off.SetCIDRs(cidrs(t, "1.2.3.0/24"), time.Now())
+	if off.Blocked("1.2.3.4") {
+		t.Error("a disabled blocklist must not block")
+	}
+	empty := NewBlocklist(true, nil)
+	if empty.Blocked("1.2.3.4") {
+		t.Error("an empty (never-loaded) blocklist must not block")
+	}
+}
+
+func TestParseDROP(t *testing.T) {
+	in := "; Spamhaus DROP\n" +
+		"1.2.3.0/24 ; SBL1\n" +
+		"  198.51.100.0/28 ; SBL2\n" +
+		"\n" +
+		"203.0.113.5 ; a bare IP -> /32\n" +
+		"2001:db8::/32 ; IPv6 skipped\n" +
+		"garbage line\n"
+	nets, err := ParseDROP(strings.NewReader(in))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(nets) != 3 {
+		t.Fatalf("got %d networks, want 3: %v", len(nets), nets)
+	}
+	bl := NewBlocklist(true, nil)
+	bl.SetCIDRs(nets, time.Now())
+	for ip, want := range map[string]bool{"1.2.3.9": true, "198.51.100.1": true, "203.0.113.5": true, "203.0.113.6": false, "2001:db8::1": false} {
+		if got := bl.Blocked(ip); got != want {
+			t.Errorf("Blocked(%s) = %v, want %v", ip, got, want)
+		}
+	}
+}
+
+func TestApplyRefresh(t *testing.T) {
+	bl := NewBlocklist(true, nil)
+	now := time.Now()
+
+	if o := ApplyRefresh(bl, cidrs(t, "1.2.3.0/24"), nil, now, time.Hour); o != RefreshUpdated {
+		t.Fatalf("success: want RefreshUpdated, got %v", o)
+	}
+	if !bl.Blocked("1.2.3.4") {
+		t.Fatal("a successful refresh must apply the feed")
+	}
+	if o := ApplyRefresh(bl, nil, errors.New("net"), now.Add(30*time.Minute), time.Hour); o != RefreshKept {
+		t.Fatalf("fresh failure: want RefreshKept, got %v", o)
+	}
+	if !bl.Blocked("1.2.3.4") {
+		t.Error("a kept feed must still block")
+	}
+	if o := ApplyRefresh(bl, nil, errors.New("net"), now.Add(2*time.Hour), time.Hour); o != RefreshDropped {
+		t.Fatalf("stale failure: want RefreshDropped, got %v", o)
+	}
+	if bl.Blocked("1.2.3.4") {
+		t.Error("a stale feed must be dropped (fail-open)")
+	}
+}