Create up to 50 time logs in one call

Each entry is validated and created independently (same rules and duplicate protection as createTimeLog), so one bad row never blocks the rest. Always returns 200 with created (each with its input index) and failed (index, code, message); check failed for partial success. A malformed request (no entries, more than 50) is a 400. Idempotent via the Idempotency-Key header.