Recompute Pricing Calibration

Manually trigger a full recompute of all Track B calibration buckets.

Equivalent to the Phase 5 backfill script. Use cases:

• Force a refresh after editing the Track A pricing config (the calibration rows reference older config versions)
• Diagnostics / cache-busting after data fixes
• Initial seeding on a fresh environment

recompute_bucket itself only flushes — this endpoint owns the transaction boundary and commits at the end so all bucket upserts persist atomically.

Auth: settings:edit.