feat: exclude per-site log/ directories from the archive

nginx access/error logs (and any other per-site log/ dir) change on
every request, so they showed up as modified in every single backup -
pure noise, no recovery value. Exclude any path containing a log/
directory (re:/log/) from borg create.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

borg-backup.sh

@@ -244,7 +244,9 @@ fi
 # MariaDB stays up throughout: the dump above is already transactionally
 # consistent, and the container's raw data directory carries a .nobackup
 # marker (excluded here via --exclude-if-present) so its live files are
-# never read by borg.
+# never read by borg. Per-site log/ dirs (nginx access/error logs etc.)
+# are excluded too - they churn on every request, so backing them up adds
+# noise and dedup overhead for no recovery value.
 step "Step 2: Creating archive $ARCHIVE"
 container_running "$DB_CONTAINER" \
     || { log "ERROR: $DB_CONTAINER is not running - refusing to archive"; exit 1; }
@@ -257,6 +259,7 @@ run_cmd timeout --signal=INT --kill-after=120s "$CREATE_TIMEOUT" \
         --exclude-caches \
         --exclude-if-present .nobackup \
         --keep-exclude-tags \
+        --exclude 're:/log/' \
         "::$ARCHIVE" \
         "$TARGET"