feat: add Withdrawn kanban column and auto rejected_date

[kbroadbent]

Summary

• Adds a Withdrawn terminal kanban column (between Rejected and Archived) for jobs you've stopped pursuing
• Adds a rejected_date field that auto-fills to today's date when a card is dragged to Rejected — no modal prompt
• Updates backend constants, model, schema, router, Alembic migration, and frontend stage config

Test Plan

• Navigate to Pipeline page — confirm Withdrawn column appears between Rejected and Archived
• Drag a card to Withdrawn — confirm it moves and persists after refresh
• Drag a card to Rejected — confirm it moves immediately (no date modal) and rejected_date is set to today in the card detail view