Rename docs/backend/*.md to Django-migration-style naming:
trello-auth-spec.md → 001_trello_auth_spec.md (Issue #1)
manage-boards-spec.md → 002_manage_boards_spec.md (Issue #2)
manage-lists-spec.md → 003_manage_lists_spec.md (Issue #3)
manage-cards-spec.md → 004_manage_cards_spec.md (Issue #4)
Each doc now includes a **Doc:** field with its sequence number.