POST /script-builder/sessions now supports origin='pilot_inline':
- Requires ai_session_id; validates it against current user ownership.
- Get-or-create: returns existing row for (user, ai_session_id) pair.
- Partial unique index on the DB backs the invariant; races resolve to
the single winner row.
list_sessions + count_user_sessions default-scope to origin='standalone'
so inline scratch sessions don't pollute the /script-builder dashboard
or count against the 5-session cap.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
