feat(backup): restore a same-version backup in place, history kept, all or nothing (#5815)

#5815 "Restore engine: in place for a same-version backup".

restore_in_place (steward/backup/restore.py) runs in one transaction:
1. It keeps aside what Steward records as it runs: ACTIVITY columns of rows
   that come back, and STATE_SETTINGS.
2. It drops every foreign key touching a replaced table.
3. It truncates the replaced tables and COPYs the backup's rows in
   unparsed, through asyncpg.
4. It puts the kept state back and moves sequences past their rows.
5. It puts right the rows whose parent didn't come back, by each key's own
   ON DELETE. A RESTRICT key that may be empty is cleared, so a run whose
   user is gone keeps its record.
6. It re-adds the keys, which checks every row.

A failure anywhere rolls the whole restore back. A backup made at another
schema is refused with NotSameSchema ("older": a full replace is the way;
"newer": update Steward).

The COPY-text reading moves into steward/backup/pgdata.py, shared by the
check and the restore. The integration tests stand in for pg_restore with
this database's own COPY output:
- settings come back while the audit log and the history of what remains
  stay;
- history of what didn't come back goes;
- a failure part-way changes nothing;
- a newer schema is refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_backup_check.py

@@ -8,8 +8,9 @@ from quart import render_template
 
 from steward.backup import check
 from steward.backup.check import (AreaCheck, CheckReport, RowDiff, TableCheck, _Opener, _Side,
-                                  compare_table, copy_rows, normalise, schema_of, script_tables,
-                                  unescape)
+                                  compare_table, normalise, schema_of)
+from steward.backup.pgdata import (copy_rows, script_blocks, script_from, script_tables,
+                                   unescape)
 from steward.core.crypto import _make_fernet
 from tests.test_style_tokens import hex_colours
 
@@ -48,6 +49,9 @@ def test_a_restore_script_gives_each_tables_rows():
     assert tables["app_settings"] == (["key", "value_json", "updated_at"],
                                       [["smtp.port", "587", "2026-10-15 14:00:00+00"]])
     assert tables["audit_events"][0] == ["id", "timestamp", "action"]
+    # Handed on unparsed, a block is the COPY text as the dump holds it.
+    assert script_blocks(script)["app_settings"][1] == b"smtp.port\t587\t2026-10-15 14:00:00+00\n"
+    assert script_tables(script_from(script_blocks(script))) == tables
 
 
 def test_an_instant_is_the_same_whatever_zone_prints_it():