fix(sync): expire delta cursors issued before a restore or a data wipe

A restore writes rows back with their original updatedAt, behind every
paired client's watermarks, and removes rows without leaving tombstones.
A client that kept its cursor would miss both on its next incremental
pull. The same holds after the person deletes all their data.

`users.sync_reset_at` (migration 0363) records when the record was last
replaced: the restore stamps it as its last write and again right after
the commit, the per-account and the admin wipe stamp it inside their
transactions. Cursors now carry the time they were issued, and
/api/sync/changes answers its existing `cursorExpired` to a cursor issued
before the reset, or one without an issue time on an account that has
one. The comparison is on the issue time, because after a restore a
freshly drained cursor still carries old watermarks.

docs/api/openapi.yaml

@@ -40212,7 +40212,8 @@ components:
           description: False once the client is caught up as of `serverNow`.
         cursorExpired:
           type: boolean
-          description: True when the supplied cursor predates tombstone retention — drop the cursor and do a clean initial sync.
+          description: True when the supplied cursor predates tombstone retention, or was issued before the account's data was
+            last restored from a backup or deleted — drop the cursor and do a clean initial sync.
         changes:
           type: object
           properties: