fix(backup): carry note_supersessions — the coverage guard caught the omission

CI failed on 45c6b1c. `tests/test_services_backup.py` asserts every table is
either in `_BACKED_UP` or explicitly in `_NOT_INCLUDED`, and the new table was
in neither.

That guard exists for exactly this: a new table gets a model and a migration —
both of which fail loudly if wrong — and then silently never gets a backup
section. No error, no warning, and a restore that reports success. Its comment
says it was written to "stop the seventh" table slipping through. This was the
seventh.

Supersession claims are backed up rather than excluded because they are a
JUDGEMENT. Someone decided this note replaced that one, and nothing in either
note's text records the decision. Lose them and a restored corpus silently
returns to ranking stale material alongside current material — with no symptom
that says why.

Scoping differs by backup kind, deliberately. A whole-instance export takes
every row. A single-user export takes only claims where BOTH ends are that
user's notes: a claim spanning out to someone else's record cannot be restored
into a single-user import, since the far id is not in the map, so exporting it
would write a row that silently vanishes on the way back in.

Restore maps both ids and skips the row unless both resolve. A claim is about a
PAIR — half of one is not a weaker claim, it is a dangling row pointing at
whatever note holds that id next. Guarded by `data.get` like every post-v2
section, so v5 and older payloads restore cleanly without it.

BACKUP_VERSION 5 -> 6, and the version test moved with it. A payload section
added without moving the version produces backups that are structurally
different and indistinguishable by inspection.

Refs #278

src/scribe/services/backup.py

@@ -7,6 +7,7 @@ from scribe.models import async_session
 from scribe.models.milestone import Milestone
 from scribe.models.note import Note
 from scribe.models.note_draft import NoteDraft
+from scribe.models.note_supersession import NoteSupersession
 from scribe.models.note_version import NoteVersion
 from scribe.models.design_system import DesignSystem, DesignToken
 from scribe.models.note_usage import NoteUsageEvent
@@ -32,8 +33,11 @@ logger = logging.getLogger(__name__)
 # when the calendar surface was retired — old v3 events are skipped on restore.
 # v5 (2026-08) added the six tables that had accumulated outside the backup
 # entirely (#2293), and the coverage guard that stops the seventh.
+# v6 (2026-08) added note_supersessions — and the guard did stop the seventh:
+# the table shipped without a backup section and the coverage test failed the
+# build, which is the whole reason that list was written.
 # Bump when the serialized schema changes.
-BACKUP_VERSION = 5
+BACKUP_VERSION = 6
 
 # Every table this backup carries, by its REAL name. Paired with _NOT_INCLUDED
 # below, these two lists must together account for the entire schema — which is
@@ -50,7 +54,7 @@ _BACKED_UP = [
     "project_topic_suppressions",
     # v5 (2026-08): the five-year gap this list was written to stop.
     "systems", "record_systems", "design_systems", "design_tokens",
-    "note_usage_events", "repo_bindings",
+    "note_usage_events", "repo_bindings", "note_supersessions",
 ]
 
 # Tables intentionally NOT in the backup, surfaced in the payload so the gap is
@@ -110,6 +114,17 @@ def _record_system_rows(rows) -> list[dict]:
     return [{"note_id": r.note_id, "system_id": r.system_id} for r in rows]
 
 
+def _note_supersession_rows(rows) -> list[dict]:
+    """Which record has overtaken which. Carried because it is a JUDGEMENT —
+    someone decided this note replaced that one, and nothing in either note's
+    text records the decision. Lose it and the corpus silently reverts to
+    ranking stale material alongside current material."""
+    return [
+        {"superseder_id": r.superseder_id, "superseded_id": r.superseded_id}
+        for r in rows
+    ]
+
+
 def _design_system_rows(rows) -> list[dict]:
     return [
         {
@@ -171,6 +186,9 @@ async def export_full_backup() -> dict:
         settings = (await session.execute(select(Setting))).scalars().all()
         systems = (await session.execute(select(System))).scalars().all()
         record_systems = (await session.execute(select(RecordSystem))).scalars().all()
+        supersessions = (
+            await session.execute(select(NoteSupersession))
+        ).scalars().all()
         # Parent-first, so a restore can resolve parent_id as it goes rather
         # than needing a second pass — the self-FK is the only ordering
         # constraint in this payload.
@@ -354,6 +372,7 @@ async def export_full_backup() -> dict:
         "design_tokens": _design_token_rows(design_tokens),
         "note_usage_events": _usage_event_rows(usage_events),
         "repo_bindings": _repo_binding_rows(repo_bindings),
+        "note_supersessions": _note_supersession_rows(supersessions),
     }
 
 
@@ -396,6 +415,17 @@ async def export_user_backup(user_id: int) -> dict:
         record_systems = (await session.execute(
             select(RecordSystem).where(RecordSystem.system_id.in_(system_ids))
         )).scalars().all() if system_ids else []
+        # BOTH ends must be this user's notes. A claim spanning out to someone
+        # else's record cannot be restored into a single-user import — the far
+        # id would not be in the map — so carrying it would export a row that
+        # silently vanishes on the way back in. Whole-instance backups have no
+        # such problem and take every row.
+        supersessions = (await session.execute(
+            select(NoteSupersession).where(
+                NoteSupersession.superseder_id.in_(note_ids),
+                NoteSupersession.superseded_id.in_(note_ids),
+            )
+        )).scalars().all() if note_ids else []
         design_systems = (await session.execute(
             select(DesignSystem).where(DesignSystem.owner_user_id == user_id)
             .order_by(DesignSystem.parent_id.nullsfirst(), DesignSystem.id)
@@ -597,6 +627,7 @@ async def export_user_backup(user_id: int) -> dict:
         "design_tokens": _design_token_rows(design_tokens),
         "note_usage_events": _usage_event_rows(usage_events),
         "repo_bindings": _repo_binding_rows(repo_bindings),
+        "note_supersessions": _note_supersession_rows(supersessions),
     }
 
 
@@ -700,6 +731,7 @@ async def _restore_v2(data: dict) -> dict:
         "topic_suppressions": 0,
         "systems": 0, "record_systems": 0, "design_systems": 0,
         "design_tokens": 0, "note_usage_events": 0, "repo_bindings": 0,
+        "note_supersessions": 0,
     }
 
     async with async_session() as session:
@@ -990,6 +1022,24 @@ async def _restore_v2(data: dict) -> dict:
             session.add(RecordSystem(note_id=mapped_nid, system_id=mapped_sid))
             stats["record_systems"] += 1
 
+        # 16b. Supersession claims. Guarded by `data.get` like every other
+        # post-v2 section, so a v5 or older payload restores cleanly without it.
+        #
+        # Both ends must map. A claim is about a PAIR — half of one is not a
+        # weaker claim, it is a dangling row pointing at whatever note happens
+        # to hold that id next.
+        for sup in data.get("note_supersessions", []):
+            mapped_new = note_id_map.get(sup.get("superseder_id", 0))
+            mapped_old = note_id_map.get(sup.get("superseded_id", 0))
+            if mapped_new is None or mapped_old is None or mapped_new == mapped_old:
+                continue
+            session.add(
+                NoteSupersession(
+                    superseder_id=mapped_new, superseded_id=mapped_old
+                )
+            )
+            stats["note_supersessions"] += 1
+
         # 17. Design systems. The export orders these parent-first, so a
         # parent's new id is always in the map by the time a child needs it —
         # no second pass, and a child whose parent is missing lands as a root

tests/test_services_backup.py

@@ -13,8 +13,11 @@ import pytest
 from scribe.services import backup
 
 
-def test_backup_version_is_v5():
+def test_backup_version_is_v6():
-    assert backup.BACKUP_VERSION == 5
+    """v6 added note_supersessions (#278). The bump is the point of the test —
+    a payload section added without moving the version produces backups that
+    are structurally different and indistinguishable by inspection."""
+    assert backup.BACKUP_VERSION == 6
 
 
 def test_not_included_lists_the_known_gaps():
@@ -102,11 +105,26 @@ async def test_export_full_backup_contains_every_declared_section():
     assert out["version"] == backup.BACKUP_VERSION
     assert out["scope"] == "full"
     assert "api_keys" in out["_not_included"]
-    # The sections v2 silently dropped, plus the six v5 added (empty here).
+    # The sections v2 silently dropped, the six v5 added, and v6's
+    # note_supersessions (all empty here).
     for key in ("rulebooks", "rulebook_topics", "rules",
                 "rulebook_subscriptions", "rule_suppressions",
                 "topic_suppressions",
                 "systems", "record_systems", "design_systems",
-                "design_tokens", "note_usage_events", "repo_bindings"):
+                "design_tokens", "note_usage_events", "repo_bindings",
+                "note_supersessions"):
         assert key in out, f"missing export section: {key}"
         assert out[key] == []
+
+
+def test_supersession_rows_serialise_the_pair():
+    """The row builder is a plain function precisely so it can be tested with
+    no database — same reason as the other v5/v6 builders."""
+    class _Row:
+        def __init__(self, a, b):
+            self.superseder_id, self.superseded_id = a, b
+
+    assert backup._note_supersession_rows([_Row(9, 4), _Row(9, 5)]) == [
+        {"superseder_id": 9, "superseded_id": 4},
+        {"superseder_id": 9, "superseded_id": 5},
+    ]