Deliverability check: each node asks what the internet sees of it

Deliverability spec (inbuxa-drafts specs/deliverability.md), the server
side. Every node that sends mail checks itself once a day, at its own
minute in the first hour (UTC), and when an administrator asks:

- its outgoing addresses (the connection strategy's, or what its EHLO
  name resolves to), their reverse DNS and whether it resolves back,
  and nine blocklists, read by each list's own codes so a refused
  query is never taken for a listing (DL-1 to DL-6);
- for every domain: SPF for each address, each DKIM key (by signing a
  message that's never sent and verifying it as a receiver would),
  DMARC, the MTA-STS policy against the MX, TLS reporting, and the
  domain blocklists (DL-7 to DL-12);
- whether it holds a certificate for its EHLO and MX names (DL-13).

It keeps one report per node, facts only; the console grades them.

- inbuxa:DeliverabilityReport: /get, and a create that asks every node
  to check now, broadcast as DeliverabilityCheck (DL-15). A tenant
  administrator gets their own domains only (DL-20).
- inbuxa:DeliverabilitySettings: which built-in lists are left out, and
  the lists themselves (DL-6).
- sysDeliverabilityGet, sysDeliverabilityUpdate, sysDeliverabilityCheck;
  a tenant ceiling always turns the last two off.

crates/common/src/ipc.rs

@@ -88,6 +88,8 @@ pub enum BroadcastEvent {
     QueueRefresh,
     // inbuxa: AL-3: end an account's open sessions on every node
     EndSessions(u32),
+    // inbuxa: deliverability spec, DL-15: every node checks itself now
+    DeliverabilityCheck,
 }
 
 #[derive(Debug, Clone, Copy)]