Add admin server status page

New page /admin/status.php gives a real-time health dashboard:

- Services panel: nginx (systemctl), php-fpm (auto-detects versioned unit names),
  site HTTP ping (curl HEAD with latency), SQLite DB (exists/writable/row count/size),
  storage directory (writable, banner/cover file counts), maintenance-mode flag.
- PHP runtime panel: version, SAPI, memory_limit, upload_max_filesize, post_max_size,
  max_execution_time.
- Disk usage bar for the partition containing APP_ROOT (colour-coded: green/amber/red).
- All shell calls go through safeExec() which suppresses stderr and checks exit code;
  systemctl/curl unavailability degrades gracefully to 'unknown' without fatal errors.
- 'Statut' nav link added to templates/admin/head.php (active state on status.php).

TODO.md

@@ -330,7 +330,7 @@ Goal: rename the tables and column to the canonical M2M pattern (`tags`, `thesis
 - [x] Add `just setup-server` recipe (rsync + run setup-server.sh on remote)
 - [x] Exclude `.claude` and `.pi` from rsync deploy
 - [x] Update `docs/SERVER_SETUP.md` with correct permissions rationale and troubleshooting
-- [ ] Add server status view in admin panel (nginx + php-fpm health, site HTTP check)
+- [x] Add server status view in admin panel (nginx + php-fpm health, site HTTP check)
 - [ ] Add server log viewer in admin panel (tail nginx error/access logs via SSH or log endpoint)
 - [ ] Add nginx config deploy flow to admin panel (upload `scripts/deploy-server.sh`, run remotely)
 - [ ] Add admin user management UI (wraps `scripts/manage-admin-users.sh` on server)