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/cmd/gateway/main.go

@@ -129,6 +129,11 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		Window:    cfg.Abuse.BanWindow,
 		Duration:  cfg.Abuse.BanDuration,
 	})
+	allow, err := parseAllowlist(cfg.Blocklist.Allow)
+	if err != nil {
+		return err
+	}
+	blocklist := ratelimit.NewBlocklist(cfg.Blocklist.Enabled, allow)
 	hub := push.NewHub(0)
 
 	var validator transcode.TelegramValidator
@@ -222,6 +227,7 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		Limiter:      limiter,
 		Tracker:      tracker,
 		Banlist:      banlist,
+		Blocklist:    blocklist,
 		Honeytoken:   cfg.Abuse.Honeytoken,
 		VKAppSecret:  cfg.VKAppSecret,
 		Hub:          hub,
@@ -243,6 +249,10 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	if cfg.Abuse.BanEnabled {
 		go runBanSync(ctx, banlist, backend, logger)
 	}
+	// When the edge blocklist is enabled (prod), keep the community feed refreshed.
+	if cfg.Blocklist.Enabled {
+		go runBlocklistRefresher(ctx, blocklist, cfg.Blocklist, logger)
+	}
 
 	public := &http.Server{Addr: cfg.HTTPAddr, Handler: edge.HTTPHandler(), ReadHeaderTimeout: readHeaderTimeout}
 	servers := []*namedServer{{name: "public", srv: public}}