feat(backup): review a restore, run it as an Activity job, verify backups

- POST /restore/preview stages an upload in the library's folder and
  answers what it holds beside the live library, writing nothing live;
  DELETE discards it; at most two stages are held (429 too-large).
- POST /restore/apply rechecks the live fingerprint (409 plan-changed) and
  starts "Database restore": exclusive, not cancellable, indeterminate. It
  waits for request leases, publishes the exact staged bytes, swaps the
  database, prunes safety backups and logs counts, repairs and the backup.
- The legacy /restore runs the same stage and publish as an exclusive hold
  with its old response, so a restore and the health check, Move library
  and every other job refuse each other.
- JobRegistry.lease: Needs review decisions and maintenance and restore
  previews are refused while an exclusive job runs, and a restore waits
  for them before it switches the file.
- GET /backup holds a guarded slot, verifies a standalone snapshot before
  streaming it and removes the closed temp file; GET /backup/status lists
  the safety backups on disk and this session's verified download.

tests/test_api_admin.py

@@ -2057,7 +2057,8 @@ class TestBackupRestoreContract(AdminAPITestCase):
         resp = await self._restore("database_file", upload)
         self.assertEqual(resp.status, 200)
         self.assertIn("verify_database_file", calls)
-        self.assertIn("restore_from_file", calls)
+        self.assertIn("stage_restore", calls)
+        self.assertIn("publish_staged", calls)
 
     async def test_restore_keeps_at_most_ten_safety_backups(self):
         import time
@@ -2098,12 +2099,12 @@ class TestBackupRestoreContract(AdminAPITestCase):
         self.api.db.save_prompt(text="restored", prompt_hash="h1")
         upload = self._snapshot_upload()
 
-        def boom(src_path):
+        def boom(staged):
             raise PermissionError(
                 13, "Permission denied", os.path.join(self.tmpdir, "prompts.db")
             )
 
-        self.api.db.model.restore_from_file = boom
+        self.api.db.model.publish_staged = boom
         resp = await self._restore("database_file", upload)
         body = await resp.text()
         self.assertEqual(resp.status, 500)