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/provisioning/alerting/rules.yaml

@@ -108,6 +108,32 @@ groups:
         labels: { severity: critical }
         annotations: { summary: 'Edge TLS cert has under 20 days left — Caddy ACME renewal may have failed.' }
 
+      # The community IP blocklist feed has not refreshed in over a day (the gateway re-fetches every
+      # few hours). Absent/NaN-safe: the age gauge appears only once a feed has loaded, so a disabled
+      # or never-fetched blocklist stays quiet. The feed itself is dropped (fail-open) at its
+      # max-staleness window; this warns well before that so the operator can fix the fetch.
+      - uid: blocklist_stale
+        title: IP blocklist feed not refreshing
+        condition: C
+        for: 15m
+        noDataState: OK
+        execErrState: OK
+        data:
+          - refId: A
+            relativeTimeRange: { from: 300, to: 0 }
+            datasourceUid: prometheus
+            model: { refId: A, expr: gateway_blocklist_age_seconds, instant: true }
+          - refId: C
+            datasourceUid: __expr__
+            model:
+              refId: C
+              type: threshold
+              expression: A
+              conditions:
+                - evaluator: { type: gt, params: [86400] }
+        labels: { severity: warning }
+        annotations: { summary: 'The community IP blocklist feed has not refreshed in over 24h — the fetch is failing; it will be dropped (fail-open) once stale. Check the gateway blocklist refresher logs and the feed URL.' }
+
   - orgId: 1
     name: scrabble-host
     folder: Alerts