Uncommitted migration work at the time of branch cleanup. Tagged as
checkpoint/m1-migration-wip so the commit survives git gc --prune=now.
Session context: Phase 3 Part B+C of the M-1 sentinel error migration
was in progress. 38 modified files, 4 new files (errors.go + errors_test.go
in internal/service/ and internal/api/handler/). Resume from this commit
via 'git checkout checkpoint/m1-migration-wip'.
shankar0123