Implement Lua scripts for atomic sequence incrementing and cold-start initialization in Redis to prevent race conditions. Improve the sequence buffer manager with CAS-like logic to prevent interval leakage during concurrent allocations. Update the message repository to use `SELECT ... FOR UPDATE` and conditional updates for `last_seq` to ensure database consistency during high concurrency.
- Add `nextSeqLua` and `initSeqLua` for atomic Redis operations
- Implement thread-safe buffer management in `seq_buffer.go`
- Add row-level locking in `message_repo.go`
- Update service layers to support group-aware sequence retrieval