`get_or_create_peers` and `get_or_create_scopes` mutate existing rows, then
insert new ones inside `db.begin_nested()`. When a concurrent writer creates
one of those rows first, the insert raises IntegrityError and the function
retries.
`begin_nested()` autoflushes the pending UPDATEs *before* opening the
savepoint, so the rollback neither undoes them nor expires the now-clean ORM
state. The retry then compared already-updated values, found no change, and
dropped those peers from `changed_peers` — skipping the cache purge while the
row change committed anyway, leaving entries stale until the 300s TTL.
Carry the mutated names into the retry via `_pending_invalidation` so the
purge cannot be lost.
The scopes facade mirrors `get_or_create_peers`, so both copies carried this.
The peer path is pre-existing and runs on every message ingest.
Also add the missing /v3 prefix to `_SCOPES_ROUTE_GUIDANCE`, which pointed
callers at a 404.
Adds tests/crud/test_get_or_create_retry_invalidation.py, which drives a real
racing session and fails without this change.
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