system: information panel and event log; about: download this build's source

System page laid out after 3CX's admin dashboard: a System information panel (version,
host name, company, public IP, LAN IPs, ports, telephony, restart needed, copy buttons,
how to restart) and an Event log panel (system_events, 90 days: trunk down/up, Asterisk
down/up, restart needed, config failed, 911, public IP changed, backups, disk) with Load
more and a CSV download.

About (/about, no sign-in, linked from the login page, My phone and System): AGPL-3.0
notice, credits, the repository link, and "Code v<version>", which downloads
selfphone.tar.gz: the api image now tars its own build context (no .env, data,
node_modules, build output or .git) before installing anything.

Task: M11-T7

docs/admin-guide/maintenance.md

@@ -97,7 +97,9 @@ curl http://127.0.0.1:8080/healthz      # 200 {"ok":true}, or 503 when the datab
 From outside, monitor `https://pbx.example.com/healthz`.
 
 **The dashboard** shows trunks, active calls, queues and warnings (emergency setup, certificate,
-backups, restart needed) live.
+backups, restart needed) live. **System** has the event log: trunks going down and back up,
+Asterisk stopping and coming back, restarts needed, 911 calls, public IP changes, backups and a full
+disk, kept 90 days (**Download** saves it as CSV).
 
 ### Keeping logs small