Host system logs source (systemd journal or /var/log)

New source, off by default and switched in Settings > Sources, that
collects the system logs of the machine hosting the stack:
- reads the systemd journal files directly (pure Go reader, no
  journalctl in the image), from /var/log/journal and /run/log/journal
  mounted read-only under /host;
- falls back to following the text files of /var/log (syslog,
  messages, *.log) on hosts without journald;
- positions saved in /data/hostlogs-state.json, HOST_LOGS_BACKFILL
  read when the source is turned on;
- source_type "host", selectable in the Source filter;
- compose mounts and group_add (HOST_LOGS_GID, adm by default), docs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.env.example

@@ -19,3 +19,9 @@ EXPORT_MAX=100000
 DOCKER_LOGS=on
 # History read from a container seen for the first time (e.g. 30m, 1h, 24h; 0 = only new lines)
 DOCKER_BACKFILL=1h
+# Host system logs (enable them in Settings > Sources)
+# Group allowed to read the host logs: 4 = adm on Debian/Ubuntu; or the systemd-journal group
+# (getent group systemd-journal | cut -d: -f3)
+HOST_LOGS_GID=4
+# History read when the source is turned on (e.g. 30m, 1h, 24h; 0 = only new entries)
+HOST_LOGS_BACKFILL=1h