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.

deploy/grafana/dashboards/service-overview.json

@@ -53,6 +53,31 @@
       "fieldConfig": { "defaults": { "unit": "bytes" }, "overrides": [] },
       "datasource": { "type": "prometheus", "uid": "prometheus" },
       "targets": [{ "refId": "A", "expr": "sum(go_memory_used) by (service_name)", "legendFormat": "{{service_name}}" }]
+    },
+    {
+      "type": "stat",
+      "title": "Blocklist entries",
+      "description": "CIDR ranges in the active community IP blocklist feed (0 when disabled or not yet fetched).",
+      "gridPos": { "h": 5, "w": 6, "x": 0, "y": 13 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "max(gateway_blocklist_entries)" }]
+    },
+    {
+      "type": "stat",
+      "title": "Blocklist feed age",
+      "description": "Seconds since the community IP blocklist feed was last successfully fetched. Grows if the fetch is failing; the feed is dropped fail-open once stale.",
+      "gridPos": { "h": 5, "w": 6, "x": 6, "y": 13 },
+      "fieldConfig": { "defaults": { "unit": "s" }, "overrides": [] },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "max(gateway_blocklist_age_seconds)" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Blocklist blocks/s",
+      "description": "Requests refused at the edge by the community IP blocklist (gateway_blocklist_blocked_total).",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 13 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(rate(gateway_blocklist_blocked_total[5m]))" }]
     }
   ]
 }