AI: Message Series planner (brief → sequence of scheduled posts) #6

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

Web behaviour: once a draft has roughly a sentence or two, the composer offers a
Message Series planner. It turns the brief into a sequence of short connected posts;
confirming creates scheduled posts spaced a few minutes apart, ready to review and adjust.

• POST /api/ai/suggest for the plan, POST /api/ai/generate to persist.
• The user reviews the plan before anything is created.
• Results are ordinary scheduled posts afterwards — editable and deletable through the
  scheduled-posts screen (which currently only supports cancel; see the composer/feed epic).

Tests: plan-parse; confirm → N scheduled messages appear in the scheduled list.

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