About: name the webmail node and the mail server node

Settings > About shows which webmail node answered (NODE_NAME, else the
container hostname) and which inbuxa node it talks to: the address the
server's name resolves to from the webmail, named by its PTR record. The
server only tells administrators its node name, so the webmail works it
out itself. Fetched from /api/about/nodes on every visit, cached for a
minute server-side, for troubleshooting a cluster.

Fork-only: upstream ihasmail runs one webmail against one server.
4 new strings, translated in all 9 catalogues.

server/src/nodes.ts

@@ -0,0 +1,68 @@
+/**
+ * ihasmail-inbuxa: which webmail node answered, and which inbuxa node it talks
+ * to -- shown in Settings > About, for troubleshooting a cluster.
+ *
+ * Not for upstream ihasmail: it only makes sense where one webmail runs per
+ * host, each pinned to its own host's mail node.
+ *
+ * The webmail node is NODE_NAME, set per host by the deploy; without it, the
+ * container's hostname, which is at least distinct. The mail node is worked
+ * out from the address the server's name resolves to *here*, where the host
+ * entry pins it to the local node, named by that address's PTR record (every
+ * node has one: mail/mx2/mx3). The server offers its node name only to
+ * administrators, so asking it would leave everyone else with nothing.
+ */
+import { lookup as dnsLookup, reverse as dnsReverse } from "node:dns/promises";
+import { hostname } from "node:os";
+import { config } from "./config.js";
+
+export interface MailNode {
+  /** The server name the webmail connects to, from its configured URL. */
+  host: string;
+  /** What that name resolves to from this container, or null if it doesn't. */
+  address: string | null;
+  /** The address's PTR name: the node's own name. Null without a PTR. */
+  name: string | null;
+}
+
+export interface Nodes {
+  webmail: string;
+  mailServer: MailNode;
+}
+
+type Lookup = (host: string) => Promise<{ address: string }>;
+type Reverse = (address: string) => Promise<string[]>;
+
+const CACHE_MS = 60_000;
+const cache = new Map<string, { node: MailNode; at: number }>();
+
+export function webmailNode(): string {
+  return config.nodeName || hostname();
+}
+
+export async function mailNode(base: string, lookup: Lookup = dnsLookup, reverse: Reverse = dnsReverse, now = Date.now()): Promise<MailNode> {
+  const host = new URL(base).hostname;
+  const hit = cache.get(host);
+  if (hit && now - hit.at < CACHE_MS) return hit.node;
+  let address: string | null = null;
+  let name: string | null = null;
+  try {
+    address = (await lookup(host)).address;
+  } catch {
+    /* unresolvable: say so rather than fail the page */
+  }
+  if (address) {
+    try {
+      name = (await reverse(address))[0]?.replace(/\.$/, "") ?? null;
+    } catch {
+      /* no PTR */
+    }
+  }
+  const node = { host, address, name };
+  cache.set(host, { node, at: now });
+  return node;
+}
+
+export function clearNodeCache(): void {
+  cache.clear();
+}