Implement backend for merge schedule end time

What does this MR do and why?

Implement backend for merge schedule end time

Changes	MR
Most backend changes	👈
Frontend & Controller	!258025

References

Related to #630505

Screenshots or screen recordings

The frontend is included in a separate MR, screenshots provided here are just for reference

These screenshots might be outdated. For up-to-date screenshots see the frontend MR.

Query for database

SELECT "merge_request_merge_schedules"."merge_after", "merge_request_merge_schedules"."merge_before", "merge_request_merge_schedules"."merge_request_id"
FROM "merge_request_merge_schedules"
WHERE
  "merge_request_merge_schedules"."merge_after" <= '2026-09-27 17:06:46.364879'
  AND ("merge_request_merge_schedules"."merge_before" IS NULL OR "merge_request_merge_schedules"."merge_before" >= '2026-09-27 17:06:46.364885')
ORDER BY "merge_request_merge_schedules"."merge_after" ASC, "merge_request_merge_schedules"."merge_before" ASC, "merge_request_merge_schedules"."merge_request_id" ASC
LIMIT 500

MR acceptance checklist

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