rclone-backup: exclude live SQLite sidecar files

The nightly sync failed every night with 'corrupted on transfer: md5
hashes differ' on gitea.db-journal: rclone copies the volatile SQLite
journal while Gitea rewrites/removes it. That IO error also made rclone
skip the delete phase, so the backup drifted out of sync. Exclude the
transient *.db-journal/-wal/-shm files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/fileserver.nix

@@ -309,6 +309,9 @@
           --tpslimit 25 \
           --drive-pacer-min-sleep 10ms \
           --retries 3 \
+          --exclude "**/*.db-journal" \
+          --exclude "**/*.db-wal" \
+          --exclude "**/*.db-shm" \
           --log-file=/var/log/rclone-backup.log \
           --log-level=INFO
       '';