fix: lock backup path during backup mutations

[giwaov]

Summary

• closes Rocksdb backup creation lockfile #3039
• adds a per-backup-directory .backup.lock file acquired with atomic create_new
• holds the lock during backup creation and purge operations so separate processes cannot mutate the same backup path concurrently

Testing

• rustfmt +stable crates/common/src/backup/manager.rs
• git diff --check
• cargo check -p citrea-common (blocked: rustup could not download pinned Rust 1.88 due DNS failure)
• cargo +stable check -p citrea-common (blocked: sparse checkout is missing unrelated workspace member crates/primitives)

[eyusufatik]

isn't this immediately dropped?

[jfldde]

holds the lock during backup creation and purge operations so separate processes cannot mutate the same backup path concurrently

This is already handled by rocksdb LOCKFILE.
Point of this issue is for observability of an on-going backup creation process