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>

web/index.html

@@ -70,6 +70,7 @@
       <option value="" data-i18n="srcAll">All sources</option>
       <option value="syslog">Syslog</option>
       <option value="docker">Docker</option>
+      <option value="host" data-i18n="srcHost">Host system</option>
     </select>
     <span class="spacer"></span>
     <span id="count" class="muted"></span>
@@ -209,6 +210,16 @@
               </div>
             </section>
 
+            <section class="set-section">
+              <h3 data-i18n="hostTitle">Host system logs</h3>
+              <p id="hostStatus" class="docker-status"></p>
+              <label class="switch-row">
+                <input id="hostEnabled" type="checkbox" class="switch">
+                <span data-i18n="hostEnabled">Collect the system logs of this machine</span>
+              </label>
+              <p class="muted small" data-i18n="hostHelp"></p>
+            </section>
+
             <section class="set-section">
               <h3 data-i18n="dockerTitle">Docker containers</h3>
               <p id="dockerStatus" class="docker-status"></p>