Export campaign deliveries as CSV

Export the per-recipient delivery outcomes of a campaign as CSV, one row per (account, channel): the loyalty account id, the Zelty customer id when the application has that integration (blank otherwise — this is the join key for cross-referencing Zelty sales exports by customer), the channel, the delivery status, the exclusion reason when filtered out, the mapped error code when failed, and the send timestamp. A campaign with no deliveries yet (draft, or an empty audience) yields a header-only CSV. For a campaign still sending, the export reflects only the deliveries recorded so far. Capped at the first 50,000 deliveries (ordered by creation time) to bound memory; larger campaigns are truncated to that many rows.