fix: support multiple workflows per form in sync import/export

Forms with multiple WorkflowDefinitions (e.g. Course Development Request
with a parent + sub-workflow) caused MultipleObjectsReturned on import
because update_or_create used form_definition as the sole lookup key.

- serialize_form() now exports all workflows (primary in 'workflow',
  extras in 'additional_workflows') with backward compatibility
- New _import_single_workflow() matches by stage name fingerprint
  instead of bare form_definition FK, preventing collisions
- Sub-workflow configs resolved in a second pass after all workflows
  exist
- Fix stage status logic: when paid=Yes, all preceding stages forced
  to approved regardless of individual SP field values
- Backfill command gains step 1b to correct pending tasks on completed
  SWIs
- migrate_sp_attachments generalized with --form flag for CIDET support

All 580 tests pass.

Author: matteius