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.

src/lib/openapi/routes/sync.ts

@@ -257,7 +257,7 @@ export const syncChangesResponse = z
     cursorExpired: z
       .boolean()
       .describe(
-        "True when the supplied cursor predates tombstone retention — drop the cursor and do a clean initial sync.",
+        "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: z.object({
       measurements: z.object({