Last window's per-org concurrency limits reverted outright in-window

What this changes for operators

• PR #14877 (merged to main 2026-06-17) reverts the DB-backed per-org/per-user concurrent sandbox/conversation limits from #14168, adding migration 124 to drop the columns introduced by migration 120. The 429-based quota some operators were waiting for is not coming in the next 1.x; the surviving concurrency-control path now counts from the runtime /list API rather than a DB flag.
• Operators who anticipated 429-based concurrency enforcement must not plan around it — it was withdrawn, not enforced.