Resume chapter generation for a story with unwritten outlines.

Detects the gap between plan.chapter_outlines and written chapters (those with non-empty text). Kicks off a background worker that writes only the missing chapters in outline order, seeding ledger context from chapters already on disk.

Use when:

• A prior generation failed or was cancelled mid-write.
• The user grew the outline post-generation through the Outline chat (refine_outline → Apply) and wants the new chapter(s) written.

No-op if every outline already has a written chapter — settles the story back at status='completed'. Refused (409) while a write or revision is already in flight.