docs: machine requirements from load, every option, 768 MB with the spam filter

Owner, 2026-10-19: "Мы рассмотрели все комбинации опций поставки? Почему со
спамом нужна машина с 1 ГБ ОЗУ? 768 МБ не хватит? 512 МБ не хватит?" The
first version measured one configuration at rest and set the host column by
estimate. Now:

- The options and how they combine: 587 (no cost at rest), port 25 (one
  listener for the relay and DMARC), the rspamd sidecar (needs port 25; an
  external rspamd costs nothing here), the proxy (host program or a
  deploy/ container).
- Measured under load on the test server (dev/server/loadmem.sh, nothing
  delivered): SelfPost 60 MB at rest, 123 MB with 20 SMTP sessions to port
  25 held open; rspamd 118 MB at rest, 153 MB scanning ten 100 KB messages
  at once, 232 MB one 25 MiB message, 424 MB three at once, 139 MB after.
  Caddy 10 MB, nginx 3 MB, Traefik 21 MB at rest (images 92/280/231 MB).
- The host column: SelfPost's peak, rspamd's for the case, and a stated
  300 MB allowance for the system and Docker (not measured on a fresh
  host). With the spam filter 768 MB when port-25 messages are small or
  rarely large, 1 GB for several 25 MiB at once; 512 MB is not enough
  (about 480 MB at rest with the allowance, +100 MB a large message).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -10,8 +10,10 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 - **Machine requirements by what is switched on** (guide § Installation,
   README § Requirements), measured on the test installation instead of the
   old unmeasured "1 vCPU, 512 MB-1 GB, 8-10 GB": 512 MB of host memory for
-  outbound mail with or without port 25, 1 GB with the spam filter; one
-  vCPU. Building the image needs 1.5 GB of memory - a build held to 1 GB was
+  outbound mail with or without port 25; with the spam filter 768 MB when
+  port-25 messages are kept small and 1 GB for several 25 MiB messages at
+  once (rspamd measured at 118 MB at rest, 232 MB on one 25 MiB message,
+  424 MB on three); the proxy containers' own; one vCPU. Building the image needs 1.5 GB of memory - a build held to 1 GB was
   killed in the Go compiler - and about 3 GB of free disk. The send log
   grows about 2-3 KB a message for its retention.