feat(manifest)!: record per volume what the run established

A finished generation cannot show whether a volume held a database, nor whether a dump was produced for it: under --only-sql a failed dump falls back to a file copy, and the resulting files/ tree looks like any other copy. The run knows both and threw the knowledge away as a printed warning, leaving every reader to guess from file names.

Each generation now carries a manifest.json stating its layout and, per volume, database / dumped / engine. baudolo.generation is the single place those names are spelled; restore/paths.py, backup/db.py and backup/volume.py stop repeating them. It is deliberately import-free so a consumer can read the manifest with nothing but json, on hosts where this package is not installed.

BREAKING CHANGE: BackupException is renamed BackupError. The rename is atomic across the ten modules that define or import it, three of which also carry the manifest change, so it lands in this commit rather than a separate one that could not import.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/backup/test_layout.py

@@ -8,7 +8,7 @@ from pathlib import Path
 from unittest import mock
 
 from baudolo.backup import layout as mod
-from baudolo.backup.shell import BackupException
+from baudolo.backup.shell import BackupError
 
 
 class TestVersionDirectory(unittest.TestCase):
@@ -21,7 +21,7 @@ class TestVersionDirectory(unittest.TestCase):
     def test_it_refuses_a_generation_another_run_already_claimed(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             mod.create_version_directory(tmp, "20260731")
-            with self.assertRaises(BackupException) as caught:
+            with self.assertRaises(BackupError) as caught:
                 mod.create_version_directory(tmp, "20260731")
             self.assertIn("20260731", str(caught.exception))