feat(lease): implement lease/TTL subsystem

[slow2342]

Summary

• Add lease management with Grant, Revoke, KeepAlive, and TimeToLive operations routed through Raft for cross-node consistency.
• Persist lease data in a dedicated RocksDB column family with atomic WriteBatch for lease + counter.
• Background expiry task detects expired leases and proposes automatic revoke.
• Enforce persist-first invariant to prevent in-memory state divergence on persistence failure.

Changes

• proto/lease.proto — Add AetherLease service definition (Grant, Revoke, KeepAlive, TimeToLive, Leases).
• src/lease/lease.rs — Lease and LeaseInfo structs with TTL/expiry semantics.
• src/lease/mod.rs — LeaseManager with in-memory state, expiry ordering, and restore from persistent storage. Load-before-clear pattern prevents partial state corruption.
• src/lease/store.rs — LeaseStore persistence layer with multi_get_cf batch optimization.
• src/api/lease.rs — gRPC LeaseService implementing AetherLease trait.
• src/raft/state_machine.rs — Apply handlers for LeaseGrant, LeaseRevoke, LeaseKeepAlive. Persist-first invariant with atomic WriteBatch for lease + counter. In-memory detach after successful batch write in Delete paths.
• src/raft/raftrs_handle.rs — Switch to std::sync::RwLock for members list with documented rationale.
• src/main.rs — Lease expiry background task, improved error logging.
• src/config.rs — Lease configuration (max_leases, max_ttl).
• src/storage/rocksdb.rs — Add lease_cf, key_lease_cf, lease_keys_cf column families.
• build.rs — Compile lease.proto.

Testing

• 29 unit tests pass covering lease grant/revoke/keep_alive, put with lease attach, revoke deletes attached keys, watch events.
• cargo clippy -- -D warnings passes with zero warnings.
• cargo fmt -- --check passes.

Related issues

Refs #79