feat(cluster): add cluster membership management

[slow2342]

Summary

Add cluster membership management support with Raft RPC layer and gRPC cluster service. This enables multi-node cluster formation, member addition/removal, and inter-node Raft communication.

Changes

• proto/raft.proto — New RaftRPC service for inter-node communication (AppendEntries, Vote, InstallSnapshot) with serde_json-encoded payloads.
• proto/cluster.proto — New AetherCluster service with MemberList, MemberAdd, MemberRemove RPCs.
• src/raft/network.rs — Replace stub network with real tonic gRPC client. Lazy connection, per-RPC timeouts (AE 5s, Vote 2s, Snapshot 60s).
• src/raft/rpc.rs — New Raft RPC server implementation that deserializes JSON payloads and delegates to the openraft instance.
• src/api/cluster.rs — New ClusterService implementing AetherCluster trait. Leadership check, auto-generated node IDs, two-step member removal.
• src/storage/rocksdb.rs — Refactor to Arc<DB>, add raft_log/raft_state column families, expose db() for shared access.
• src/main.rs — Wire up full Raft lifecycle: log store, state machine, network, bootstrap, register all three gRPC services.
• src/error.rs — Add NotLeader and Raft error variants.
• build.rs — Compile all three proto files.
• Cargo.toml — Add async-trait dependency.

Testing

• cargo build — passes
• cargo test — 10 tests pass
• cargo clippy -- -D warnings — no warnings
• cargo fmt -- --check — passes

Related issues

Refs #77