Add API-level campaign delivery window pacing

[esokullu]

Summary

This adds campaign-level delivery window pacing so a single campaign can be spread evenly over a configured time window without manually splitting a list into many smaller campaigns.

New API fields:

• send_until: absolute timestamp that marks the end of the delivery window.
• send_window: convenience duration such as 24h; it is converted to send_until relative to send_at, or to the current time when send_at is empty.

Example:

{
  "send_at": "2026-07-14T10:00:00Z",
  "send_window": "24h"
}

The campaign remains one campaign in the UI/API. It does not create child campaigns or separate drip records.

Why

listmonk already has global throughput and sliding-window rate limits, which are useful for server/provider limits. This patch targets a different use case: per-campaign pacing for sender reputation, warmup, and scheduled newsletter delivery.

Without this, users who want to send one campaign over 12-24 hours have to manually create list segments and schedule many separate campaigns. That fragments analytics and is easy to misconfigure.

Implementation Notes

• Adds nullable campaigns.send_until with a migration.
• Exposes send_until on campaign models and API responses.
• Adds send_window as an API-only helper field for create/update requests.
• Updates campaign validation so send_until must be after send_at or now.
• Paces fetching subscribers in the campaign pipe. This avoids advancing last_subscriber_id ahead of unsent work and keeps progress on one campaign row.
• Existing behavior is unchanged when send_until / send_window are not set.

Testing

• Added focused unit tests for paced batch allowance calculation.
• git diff --check passes locally.
• I could not run the full Go test suite in my local environment because Go was not installed there; CI should verify the full build/test matrix.

Follow-up

This is intentionally backend/API-first. If the approach is acceptable, a follow-up can add UI controls such as "Send immediately" vs "Spread over..." to the campaign form.