docs(guide,architecture): move internal env vars out of the operator guide

Internal (non-operator) env vars were listed in guide.md's Environment
variables section even though the guide's own text says operators
aren't meant to touch them. They now live in architecture.md §
Configuration, the as-built/internals doc; guide.md keeps a one-line
pointer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/guide.md

@@ -196,33 +196,10 @@ TLS certificate paths (`TLS_CERT_FILE`, `TLS_KEY_FILE`) are fixed in
 [deploy/docker-compose.yml](../deploy/docker-compose.yml) to match the `./certs`
 bind mount — configure the mount, not these variables.
 
-**Internal variables (not part of the operator interface).** The following are
-read by the panel or startup scripts but are not meant to be changed in a
-normal deployment; documenting them here avoids treating accidental overrides as
-supported configuration:
-
-- **Panel paths and tuning:** `SELFPOST_DATA_DIR` (`/data`), `SELFPOST_DB_PATH`
-  (`/data/selfpost.db`), `SELFPOST_SETUP_TOKEN_FILE`
-  (`/data/setup-token`), `PANEL_HTTP_ADDR` (`:8080`),
-  `JOURNAL_MILTER_SOCKET` (`/run/selfpost/journal.sock`), `MAIL_LOG`
-  (`/data/log/mail.log` — read by the panel and written by Postfix, so a change
-  here has to be matched in `build/postfix-config.sh`),
-  `PANEL_COOKIE_SECURE` (`true`), `OPENDKIM_SOCKET`
-  (`/run/opendkim/opendkim.sock`), `OPENDKIM_DIR` (`/data/opendkim`),
-  `DKIM_SELECTOR_DEFAULT` (`selfpost`), `SASL_DB_PATH`
-  (`/data/sasl/sasldb2`), `SASL_REALM` (defaults to `SELFPOST_HOSTNAME`),
-  `POSTFIX_DIR` (`/data/postfix`), `POSTFIX_SENDER_LOGIN_MAPS`
-  (`/data/postfix/sender_login_maps` — read by Postfix config only; the panel
-  always writes `<POSTFIX_DIR>/sender_login_maps`, so overriding this env alone
-  desyncs the map Postfix reads from the file the panel maintains).
-- **Milter and Postfix startup:** `MILTER_CONNECT_TIMEOUT` (`15s`),
-  `MILTER_COMMAND_TIMEOUT` (`15s`), `MILTER_CONTENT_TIMEOUT` (`30s`),
-  `MILTER_WAIT_TIMEOUT` (`30` seconds).
-- **Background maintenance:** `TLS_RELOAD_INTERVAL_SECONDS` (`86400` — daily
-  `postfix reload` to pick up renewed certificates),
-  `LOGROTATE_INTERVAL_SECONDS` (`21600` — check `mail.log` rotation every six
-  hours; logrotate keeps 14 rotated files on a daily schedule, and each
-  rotation triggers `postfix reload`).
+The image also reads a number of internal, non-operator env vars (paths,
+timeouts, tuning) — not part of this interface; see
+[architecture.md § Configuration](architecture.md#configuration) if you need
+them.
 
 ### Reverse proxy (mandatory)