Reprocess Attachment

Re-run the full attachment pipeline on an existing attachment.

Two use cases:

1. Recover a 'failed' row — extraction or a later step crashed, leaving the attachment in a permanent error state. Reprocess re-extracts, re-classifies, and re-indexes from scratch.
2. Re-run after schema change — template goals or classifier behavior changed and the attachment's file_role / document_kind should be recomputed.

The endpoint flips status to 'processing', runs the same _process_attachment background-task body the upload path uses, and returns the freshly-updated file_role + document_kind + form_structure.