Adds enginesExplicit to the strict compression PUT schema so the panel can save without a 400. Kept only this commit; the Hub-render commit was superseded by Phase 2 (#4521) which rewrote CompressionHub.tsx. Does not reopen the Phase 1 gate (enginesExplicit is recomputed from stored engines, the persisted row is ignored). Tested.
Integrated into release/v3.8.33.