[codex] refactor: add SnapshotId alias

[Herrtian]

Summary

• Add a SnapshotId alias alongside the existing table, partition, and bucket ID aliases.
• Update hand-written snapshot metadata, admin, and request APIs to use SnapshotId instead of bare i64.
• Leave protobuf wire fields as int64.

Validation

• cargo fmt --check
• git diff --check
• cargo check -p fluss-rs
• cargo test -p fluss-rs metadata::kv_snapshot --lib

Closes #636