Follow-ups from the main merge:

- Migration numbering: main's 035_ProjectionThreadTitleRegeneration is
  released and keeps its id; this branch's unreleased v2 migrations shift
  to 036-044 instead (a released migration can never be renumbered).
- thread.metadata.update accepts regenerateTitle: true arms an in-flight
  titleRegeneration marker on the thread payload (requestId + startedAt),
  a landing title or explicit false clears it. The marker projects onto
  thread shells and through the client-runtime shell model, so the
  sidebar's Regenerating state works unchanged.
- New ThreadTitleRegenerationService worker reacts to armed markers on
  the live domain-event stream (so ws, MCP, and mobile dispatches all
  behave the same): builds a newest-first conversation digest from the
  v2 projection (8k-char budget, retained attachments — ported from the
  v1 reactor), generates via TextGeneration, and lands the title with a
  follow-up metadata update. Failures clear the marker and log.
- The server advertises threadTitleRegeneration again, and the client
  updateThreadMetadata dispatches regenerateTitle-only updates.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>