feat: replace tar+DB+LND backup with ext4+rsync workflow

- Rewrite sovran-hub-backup.sh: rsync-based mirror to stable current/
  path, ext4 validation, no tar/pg_dump/mariadb-dump/LND orchestration,
  exit-24 nonfatal for /home, INCOMPLETE/BACKUP_COMPLETE markers, flock
- Update server.py: _is_supported_backup_fstype accepts only ext4
- Update support.js: require ext4, explain database limitations, update
  failure message from exFAT to ext4
- Update sovran-hub.nix: replace pkgs.gnutar with pkgs.rsync + pkgs.acl
- Rewrite test_manual_backup_workflow.py: 40 new tests covering rsync
  options, ext4 acceptance, exit-24 handling, no-delete, stable current/
  path, INCOMPLETE markers, behavioral rsync tests, and regressions

app/sovran_systemsos_web/static/js/support.js

@@ -491,8 +491,9 @@ function renderBackupReady(drives) {
       '<div class="support-steps-title">Requirements</div>',
       '<ol class="support-backup-steps">',
         '<li>USB hard drive plugged into one of the open USB ports on your Sovran Pro</li>',
-        '<li>Enough free space for your selected backup data (the backup checks this before starting)</li>',
-        '<li>Drive must be formatted as <strong>exFAT</strong></li>',
+        '<li>Enough free space for your data (the backup checks this before starting)</li>',
+        '<li>Drive must be formatted as <strong>ext4</strong> (a Linux filesystem). Drives with exFAT, FAT32, or NTFS are not supported. To format a drive as ext4, use a Linux tool such as GParted or <code>mkfs.ext4</code> — note that formatting erases all data on the drive.</li>',
+        '<li>The drive is intended for Linux/Sovran recovery. It may not be directly readable by Windows or macOS without additional software.</li>',
       '</ol>',
     '</div>',
 
@@ -501,17 +502,25 @@ function renderBackupReady(drives) {
       '<ol class="support-backup-steps">',
         '<li>NixOS configuration (<code>/etc/nixos</code>)</li>',
         '<li>nix-bitcoin secrets (<code>/etc/nix-bitcoin-secrets</code>)</li>',
-        '<li>System service data (<code>/var/lib</code>) including Vaultwarden, bitcoind, LND, sovran-hub, domains, and secrets</li>',
+        '<li>System service data (<code>/var/lib</code>) — excluding databases and blockchain data (see note below)</li>',
         '<li>Home directory (<code>/home</code>)</li>',
       '</ol>',
     '</div>',
 
     '<div class="support-wallet-box support-wallet-warning">',
+      '<div class="support-wallet-header">',
+        '<span class="support-wallet-icon">\u2139\ufe0f</span>',
+        '<span class="support-wallet-title">Database and Blockchain Data</span>',
+      '</div>',
+      '<p class="support-wallet-desc">Application databases stored in PostgreSQL or MariaDB/MySQL are <strong>not included</strong> in Manual Backup. Bitcoin blockchain and Electrs index data are also excluded (they are stored on the internal second drive). If you use Nextcloud, Matrix, or other database-backed applications, back up those databases separately with their native tools.</p>',
+    '</div>',
+
+    '<div class="support-wallet-box support-wallet-protected">',
       '<div class="support-wallet-header">',
         '<span class="support-wallet-icon">\u23f1\ufe0f</span>',
         '<span class="support-wallet-title">Time Estimate</span>',
       '</div>',
-      '<p class="support-wallet-desc">This backup can take <strong>up to 4 hours</strong> depending on the amount of data stored on your Sovran Pro and the speed of your external hard drive. Be patient\u2026</p>',
+      '<p class="support-wallet-desc">The first backup may take a while depending on how much data you have. Later backups are much faster because only changed or new files are copied. Files are stored directly on the drive and can be browsed without any special software.</p>',
     '</div>',
 
     driveSelector,
@@ -619,7 +628,7 @@ function renderBackupDone(success) {
       '<div class="support-section">',
       '<div class="support-icon-big">\u26a0\ufe0f</div>',
       '<h3 class="support-heading">Backup Failed</h3>',
-      '<p class="support-desc">The backup did not complete successfully. Please check that the USB drive is still connected, has enough free space, and is formatted as exFAT. Then try again.</p>',
+      '<p class="support-desc">The backup did not complete successfully. Please check that the USB drive is still connected, has enough free space, and is formatted as ext4. Then try again.</p>',
       '<div class="modal-log" id="backup-log-fail" style="text-align:left;"></div>',
       '<button class="btn support-btn-done" id="btn-backup-close">Close</button>',
       '</div>',