Add local rspamd and redis to the mail edge gateway (#553)

Edge gateways need cheap deterministic in-session filtering behind the postfix milter, with no AI and no ClamAV on 1GB VMs.

- add a signature-checked rspamd yum repo via artifactapi, enabled for the gateway role
- add profiles::rspamd::edge managing the package and local.d files from hiera
- fetch rule files without blocking the service: empty placeholder, swap only on a successful changed fetch, then reload
- run a self-scanning milter proxy on 11332; reject at 13, header at 6, no greylisting, subject rewrite or gpt
- add a local 128MB redis for bayes autolearn; load SpamAssassin rules from rspamd-rules
- bypass rspamd for mail relayed out by the smarthost

Stacked on #552; requires terraform-artifactapi #63 applied first.

Reviewed-on: #553
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

hieradata/roles/infra/mail/gateway.yaml

@@ -1,4 +1,58 @@
 ---
+hiera_include:
+  - redis
+
+profiles::yum::global::repos:
+  rspamd:
+    ensure: present
+
+# local redis for rspamd bayes and history; bayes tokens carry a ttl so volatile-ttl can evict
+redis::maxmemory: '128mb'
+redis::maxmemory_policy: 'volatile-ttl'
+
+# local rspamd, deterministic checks only; module defaults cover spf/dkim/dmarc/arc/rbl/phishing/mime_types/fuzzy_check
+profiles::rspamd::edge::rules:
+  edge.cf: 'https://git.unkin.net/unkin/rspamd-rules/raw/branch/main/edge.cf'
+profiles::rspamd::edge::local_d:
+  worker-proxy.inc: |
+    bind_socket = "127.0.0.1:11332";
+    milter = yes;
+    timeout = 120s;
+    count = 1;
+    upstream "local" {
+      default = yes;
+      self_scan = yes;
+    }
+  worker-normal.inc: |
+    enabled = false;
+  redis.conf: |
+    servers = "127.0.0.1:6379";
+  actions.conf: |
+    reject = 13;
+    add_header = 6;
+    rewrite_subject = null;
+    greylist = null;
+  greylist.conf: |
+    enabled = false;
+  gpt.conf: |
+    enabled = false;
+  classifier-bayes.conf: |
+    backend = "redis";
+    expire = 8640000;
+    autolearn {
+      spam_threshold = 6.0;
+      ham_threshold = -0.5;
+      check_balance = true;
+    }
+  spamassassin.conf: |
+    ruleset = "/etc/rspamd/rules/*.cf";
+    alpha = 0.1;
+  settings.conf: |
+    smarthost_outbound {
+      priority = high;
+      ip = "103.216.191.185";
+      want_spam = yes;
+    }
 
 # base postfix configuration (passed to postfix class)
 postfix::relayhost: 'direct'