Create Invitations Batch

Invite up to 100 emails at once with a common org role + common per-project roles (task #2089). Best-effort against the seat cap: emails are invited in order until the cap is hit, the rest returned skipped: seat_limit. Reuses the single-invite logic; results are reported per email. No tokens in the response — accept links go by email.

Fail-fast (whole request): a bad org/project role → 403/422 before anything is created. Non-fatal per email: already a member / already invited / a duplicate in the payload → skipped.