Identify and implement top 5 code reduction

Identify top 5 opportunities to massively reduce the amount of code (lines of code) in any part of the repo: - Similar functionality that can be unified and reused - Code that has been valuable but no more - A front end component or a group of them duplicate another component/page functionality. Use the most modern and nice looking - etc. Now implement them :)

Surveyed the whole repo with 6 detectors + 5 recon sweeps: no large duplicate or dead region exists. Folded the 3 real copies (slug allocation, counted envelope, commit-or-409) in apps/api; PR #7559 queued to the merge lane, quick gate + 787 tests green. Deferred: generating apps/web/src/types.ts. No fifth fold survived; no UI surface affected. Memory: none applied.