AI: Article Series planner (brief → folder of documents) #7

Part of #3 — AI Writing Assistance is entirely missing from the Android app

Web behaviour: the composer's Article Series planner turns a brief into a set of
related documents. Generating creates a folder with one document per part, in order.

• POST /api/ai/suggest → reviewable outline; POST /api/ai/generate → folder + documents.
• The created folder and documents must show up in the existing documents folder browser
  without a manual refresh.

Tests: outline parse; confirm → folder + ordered documents created and visible in the
browser.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.