feat(cluster): add maintenance service with defrag, alarm, and status

[slow2342]

Summary

Add AetherMaintenance gRPC service providing operational tools for cluster maintenance: Defrag, Alarm, and Status.

Changes

• proto/maintenance.proto — Service definition with Defrag, Alarm, Status RPCs, AlarmType enum (NONE, NOSPACE, CORRUPT), AlarmAction enum (GET, ACTIVATE, ACKNOWLEDGE).
• src/cluster/alarm.rs — Thread-safe AlarmManager with activate, acknowledge, get_all, has_alarm. 7 unit tests.
• src/cluster/mod.rs — Cluster module entry, exports AlarmManager and AlarmType.
• src/api/maintenance.rs — MaintenanceService implementing all 3 RPCs. defrag runs via spawn_blocking. alarm validates action/alarm_type and delegates to AlarmManager. status returns real raft term, commit/applied index, db sizes, and dynamic learner status.
• src/storage/rocksdb.rs — defrag() compacts only default and mvcc CFs. approximate_size() uses rocksdb.total-sst-files-size. approximate_mem_usage() uses rocksdb.estimate-live-data-size.
• src/raft/node.rs — Added term: AtomicU64 to RaftSharedState, updated on every tick and ready cycle.
• src/raft/handle.rs — Added fn term(&self) -> u64 to RaftHandle trait.
• src/raft/raftrs_handle.rs — Implemented term() reading from shared state.
• src/main.rs — Wired AlarmManager and MaintenanceService into gRPC server.
• build.rs — Added proto/maintenance.proto to compilation.

Testing

• 102 unit tests pass (including 7 new alarm tests)
• cargo clippy -- -D warnings — no new warnings (pre-existing in raftrs_store.rs)
• cargo fmt -- --check — clean

How to verify

cargo build
cargo test -p aether --lib
cargo clippy --all-targets -- -D warnings
cargo fmt -- --check

Related issues

Refs #88