docs: Add ARCHITECTURE.md with diagram of components

ARCHITECTURE.md

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+This diagram shows components of the chatmail server; this is a draft
+overview as of mid-August 2025:
+
+```mermaid
+graph LR;
+    cmdeploy --- sshd;
+    letsencrypt --- |80|acmetool-redirector;
+    acmetool-redirector --- |443|nginx-right(["`nginx
+    (external)`"]);
+    nginx-external --- |465|postfix;
+    nginx-external(["`nginx
+    (external)`"]) --- |8443|nginx-internal["`nginx
+    (internal)`"];
+    nginx-internal --- website["`Website
+    /var/www/html`"];
+    nginx-internal --- newemail.py;
+    nginx-internal --- autoconfig.xml;
+    certs-nginx[("`TLS certs
+    /var/lib/acme`")] --> nginx-internal;
+    cron --- chatmail-metrics;
+    cron --- acmetool;
+    cron --- expunge;
+    chatmail-metrics --- website;
+    acmetool --> certs[("`TLS certs
+    /var/lib/acme`")];
+    nginx-external --- |993|dovecot;
+    autoconfig.xml --- postfix;
+    autoconfig.xml --- dovecot;
+    postfix --- echobot;
+    postfix --- |10080,10081|filtermail;
+    postfix --- users["`User data
+    home/vmail/mail`"];
+    postfix --- |doveauth.socket|doveauth;
+    dovecot --- |doveauth.socket|doveauth;
+    dovecot --- users;
+    dovecot --- |metadata.socket|chatmail-metadata;
+    doveauth --- users;
+    expunge --- users;
+    chatmail-metadata --- iroh-relay;
+    certs-nginx --> postfix;
+    certs-nginx --> dovecot;
+    style certs fill:#ff6;
+    style certs-nginx fill:#ff6;
+    style nginx-external fill:#fc9;
+    style nginx-right fill:#fc9;
+```
+
+The edges in this graph should not be taken too literally; they
+reflect some sort of communication path or dependency relationship
+between components of the chatmail server.