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/server.py

@@ -1494,38 +1494,13 @@ def _is_internal_mount(mnt: str) -> bool:
 
 
 def _is_supported_backup_fstype(path: str, fstype: str) -> bool:
-    """Return whether the target filesystem type is supported for manual backup."""
+    """Return whether the target filesystem type is supported for manual backup.
+
+    Manual Backup requires ext4 for Linux metadata preservation (ACLs, xattrs,
+    hard links). exFAT, FAT32, NTFS, and other filesystems are not supported.
+    """
     fstype = (fstype or "").lower()
-    if fstype == "exfat":
-        return True
-    if fstype != "fuseblk":
-        return False
-
-    src_dev = ""
-    try:
-        result = subprocess.run(
-            ["findmnt", "-n", "-o", "SOURCE", "-T", path],
-            capture_output=True, text=True, timeout=5,
-        )
-        if result.returncode == 0:
-            src_dev = result.stdout.strip()
-    except Exception:
-        src_dev = ""
-
-    if not src_dev:
-        return False
-
-    for cmd in (
-        ["lsblk", "-no", "FSTYPE", src_dev],
-        ["blkid", "-o", "value", "-s", "TYPE", src_dev],
-    ):
-        try:
-            result = subprocess.run(cmd, capture_output=True, text=True, timeout=5)
-            if result.returncode == 0 and result.stdout.strip().lower() in {"exfat", "fuseblk"}:
-                return True
-        except Exception:
-            continue
-    return False
+    return fstype == "ext4"
 
 
 def _detect_external_drives() -> list[dict]:
@@ -3799,7 +3774,7 @@ async def api_backup_run(target: str = ""):
     if selected_fstype and not _is_supported_backup_fstype(selected_target, selected_fstype):
         raise HTTPException(
             status_code=400,
-            detail=f"Selected drive filesystem '{selected_fstype}' is not supported for manual backup.",
+            detail=f"Selected drive filesystem '{selected_fstype}' is not supported for manual backup. Manual Backup requires an ext4-formatted drive.",
         )
 
     # Clear stale log before starting