Robust pipeline stage to status mapping (no silent empty columns)

[RikepilB]

Problem

src/app/api/admin/jobs/[id]/pipeline/route.ts groups applicants by stage.name.toUpperCase() matched against the ApplicationStatus enum (NEW, REVIEWING, SHORTLISTED, INTERVIEW, OFFERED, REJECTED, WITHDRAWN). User-defined stage names that don't match an enum value silently produce empty columns. The seed stages (New / Screening / Interview / Offer / Hired) only match NEW and INTERVIEW — the rest render empty.

Fix

Decouple display stages from the status enum. Options (pick in implementation):

• Store an explicit status mapping on each PipelineStage, OR
• Persist applicant→stage by stage id rather than deriving from name→enum.
• Backfill existing data; ensure no applicants are dropped from view.

Acceptance

• Custom stage names show their applicants (no silent empty columns)
• Seed pipeline renders all stages populated correctly
• Existing applicants preserved after migration
• Test covering non-enum stage names

Notes

Documented in .claude/rules/scoutlane-architecture.md and CLAUDE.md as a known coupling.