fix(telemetry): the reading project survives a backup (#4196)

Run 7068 failed two tests on a4aae97, and the second was a real bug the
guard caught before it shipped.

THE COLUMN GUARD (#3182) was right. `note_usage_events` excludes only
`id` from backup, so telemetry IS exported — and a new column that the
serialiser never learned about would have restored as null on every row.
That is the exact failure #3182 was built from, where nine columns
vanished from `notes` the same way: added to the model and the migration,
which fail loudly, and never to the serialiser, which fails silently.

So `project_id` is exported, not excluded, via the one `_usage_event_rows`
builder both exporters share.

The importer re-maps it through `project_id_map` and DEGRADES rather than
skipping. `code_shape_events` drops a row whose project will not map,
because a shape event without its project says nothing; a usage event is
not like that. The project is optional by design and null already means
"not reported", so an unmappable one restores as unreported. Dropping it
would lose a real pull and deflate the pull-through this table exists to
report.

BACKUP_VERSION deliberately unchanged: the suite states that column
additions do not bump it, only new sections do, and a third test asserts
the current number.

The second failure was mine and smaller — test_record_pulled_writes_a_
single_row compares exact dicts, so it now expects the null project. I
had claimed these assertions were key-based; they are not, and I had
read a summary line rather than the assertion.

Added an import-side guard while here. The column family checks the
EXPORT side only, so a column can be exported faithfully and dropped on
the way back in — a restore that reports success and has quietly lost a
dimension. No general guard for that direction exists yet; this covers
the column added here and names the gap for whoever builds the general
one.

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

src/scribe/services/backup.py

@@ -333,7 +333,7 @@ def _usage_event_rows(rows) -> list[dict]:
     return [
         {
             "user_id": r.user_id, "note_id": r.note_id, "event": r.event,
-            "source": r.source,
+            "source": r.source, "project_id": r.project_id,
             "created_at": r.created_at.isoformat() if r.created_at else None,
         }
         for r in rows
@@ -1548,6 +1548,12 @@ async def _restore_v2(data: dict) -> dict:
                 note_id=mapped_nid,
                 event=ev.get("event", ""),
                 source=ev.get("source", ""),
+                # Degrades to None rather than dropping the row: unlike a shape
+                # event, a usage event without a project is still a real pull,
+                # and discarding it would deflate the pull-through this table
+                # exists to report. Null already means "not reported".
+                project_id=(project_id_map.get(ev["project_id"])
+                            if ev.get("project_id") else None),
                 created_at=_dt(ev.get("created_at")),
             ))
             stats["note_usage_events"] += 1