As an Authenticated User, I want a failed page retried so that one bad call does not stop the document

[RomanNabukhotniidev]

Retries with a delay for transient failures (provider timeouts), a repair request for malformed output, and a terminal failed state when attempts run out.

Endpoint: POST /api/v1/pages/:id/reprocess to re-read a page on demand. See docs/02-data-pipeline.md.