Sync Users Dump

Full-table user dump for cross-region replication.

The downstream CN deployment polls this every 60s and upserts each row into its local users table (US-wins). Pagination intentionally omitted — at the current 130-row scale the entire payload fits in a single ~65 kB JSON response, and any future delta path (since=<iso>) would still need a fallback "full reconcile" mode which is exactly what this endpoint is. When the table approaches 10⁴ rows, add a ?since= filter against an updated_at column.