Disk buffer for batches VictoriaLogs cannot take, lossless Docker positions, non-blocking reverse DNS

- Batches that fail go to /data/spool (SPOOL_MAX_MB, 1 GiB by default) and
  are sent again oldest first; retries no longer block the store loop and
  follow the shutdown context.
- Docker and host logs wait for room in a full queue instead of being
  dropped; the Docker position only moves once a line is stored or spooled.
- Reverse DNS no longer holds up the syslog listeners, with an LRU cache
  and a cap on concurrent lookups.

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

web/style.css

@@ -452,6 +452,7 @@ mark.hit { background: var(--hit); color: inherit; border-radius: 3px; padding:
 .conn.ok::before { color: var(--sev-info); }
 .conn.ko::before { color: var(--sev-err); }
 #stats .bad { color: var(--sev-err); }
+#stats .warn { color: var(--sev-warning); }
 /* "Back to top", at the right end of the status bar: never over a log row */
 .to-top { width: 24px; height: 24px; margin: -5px -6px -5px auto; flex: none; } /* no taller bar */
 .to-top svg { width: 16px; height: 16px; }