test(backup): prove the three restored code_shapes columns survive real Postgres (#4197)

The column guard is a claim about kwargs; "the value comes back" is a
different claim, and only a round trip settles it. This drives the real
`restore_full_backup` over a judged shape carrying all three of the columns
that were going missing.

`diverges_from` gets the harder assertion. It is a FK to notes.id, so the
tempting fix — carry the exported id across — produces a row pointing at
whatever note holds that number in the target database: not dropped,
REATTACHED, with the restore reporting success and the divergence about the
wrong snippet. So the test asserts WHOSE note the pointer landed on rather
than which integer it holds, and it refuses to run at all if the restore
happened to reuse the source id, which would let it pass without proving
anything. Same shape as the assertion in the rule_usage round trip, for the
same seam.

Also corrects this module's own docstring, which said the round-trip module
"is not written yet". Three existed; this is the fourth.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_services_backup.py

@@ -2,10 +2,14 @@
 
 This is the no-database lane, so these cover the parts that need none: the
 version/coverage constants, the pure row helpers, and the export dict shape
-(via a mocked session). The full FK-remapping round-trip needs real Postgres
-and belongs in a `@pytest.mark.integration` module — it is not written yet,
-which is why every row helper here is a plain function that can be tested
-without a session.
+(via a mocked session), plus the two COLUMN GUARDS — one per direction — which
+need only a stand-in and a builder.
+
+The full FK-remapping round-trip needs real Postgres and lives in the
+`test_integration_backup_*_roundtrip.py` modules, which drive the real
+`restore_full_backup`. That sentence used to read "it is not written yet";
+four of those files exist now, the newest covering the three `code_shapes`
+columns the import guard below found missing.
 """
 from datetime import datetime, timezone
 from types import SimpleNamespace