install/systemd: tglist-sync timer for follower boxes; INSTALL.md notes on systemd/cron

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

INSTALL.md

@@ -144,6 +144,34 @@ sudo systemd-tmpfiles --create /etc/tmpfiles.d/svxportal-tglist.conf
 This creates `/etc/svxlink/tglist.json` owned by `www-data`. The path is
 `tglist_file` in `config.php`; change both if you want it elsewhere.
 
+### 6b. Optional: follower box that mirrors another portal's talkgroup list
+
+If you run several reflectors and want ONE talkgroup list, one portal is the
+master (its list is edited in the web) and the others pull it four times a day
+with a systemd timer. Nothing but `curl` and `python3` is involved.
+
+```bash
+sudo cp install/systemd/svxportal-tglist-sync.service install/systemd/svxportal-tglist-sync.timer /etc/systemd/system/
+sudo cp install/systemd/tglist-sync.sh /etc/svxlink/tglist-sync.sh && sudo chmod 755 /etc/svxlink/tglist-sync.sh
+sudo cp install/systemd/default-svxportal-tglist-sync /etc/default/svxportal-tglist-sync
+sudoedit /etc/default/svxportal-tglist-sync      # UPSTREAM = the master's https://.../tglist.php
+sudo systemctl daemon-reload
+sudo systemctl enable --now svxportal-tglist-sync.timer
+sudo systemctl start svxportal-tglist-sync.service && journalctl -u svxportal-tglist-sync -n 3
+```
+
+On the master, or on a standalone box, do **not** install this (or leave
+`UPSTREAM` empty; the script then exits without touching the file).
+
+Other systemd notes:
+
+- `svxreflector.service` is used **unmodified** from the SvxLink package. No
+  drop-in is needed for the portal.
+- Redis runs from Debian's `redis-server@.service` template (step 3); no unit
+  file to add.
+- There are no cron jobs. PHP session cleanup is Debian's stock
+  `phpsessionclean.timer`.
+
 ## 7. nginx + TLS
 
 ```bash