0.33.0: NFS share as backup destination

New destination type 'nfs' alongside SFTP and S3. The StackPilot container
needs no mount privileges: the Docker daemon mounts the export as a named
volume (stackpilot-nfs-dest-<id>, driver local/type nfs, recreated whenever
server/path/options change) and all file I/O runs through throwaway helper
containers (BACKUP_HELPER_IMAGE) — upload via put_archive, list via stat,
download via get_archive, delete/test via short-lived runs. Config: server,
export path, mount options (default rw), optional subdirectory (sanitized;
shell-safe charset). Mount failures surface as clean destination errors.

Settings UI gains the NFS form + summary; works everywhere destinations are
used (push, restore-from, scheduled backups incl. retention).

Verified live against a real kernel NFS server: test, push (file on the
export), list, restore-from incl. volume data, remote delete, config change
recreates the mount volume, unreachable server fails cleanly.

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

backend/models/backup_destination.py

@@ -10,14 +10,14 @@ def _now() -> datetime:
     return datetime.now(timezone.utc)
 
 
-DESTINATION_TYPES = ["sftp", "s3"]
+DESTINATION_TYPES = ["sftp", "s3", "nfs"]
 
 # config keys that hold secrets — masked in API responses.
 SECRET_KEYS = {"password", "private_key", "secret_key"}
 
 
 class BackupDestination(SQLModel, table=True):
-    """A remote target for stack backups (SFTP or S3-compatible)."""
+    """A remote target for stack backups (SFTP, S3-compatible or NFS)."""
 
     id: Optional[int] = Field(default=None, primary_key=True)
     name: str