feat(api): add gRPC API layer with proto definitions and KV service

[slow2342]

Summary

• Define protobuf types for KV operations in proto/common.proto and proto/kv.proto
• Wire up tonic-prost-build for proto code generation
• Implement AetherKv gRPC service (Put/Get/Delete/Range) backed by RocksDB
• Add range_scan method to StorageEngine trait for range queries
• Start gRPC server in main.rs
• Txn returns unimplemented for now

Changes

• proto/common.proto — Shared types: ResponseHeader, KeyValue, SortOrder, SortTarget, WatchEvent, LeaseInfo
• proto/kv.proto — AetherKV service with 5 RPCs and transaction types
• build.rs — Proto compilation via tonic-prost-build
• src/proto/mod.rs — Includes generated proto code
• src/api/mod.rs — API module entry
• src/api/kv.rs — KvService implementing AetherKv trait
• src/storage/mod.rs — Added range_scan to StorageEngine trait
• src/storage/rocksdb.rs — Implemented range_scan using RocksDB iterator
• src/lib.rs — Added api and proto modules
• src/main.rs — gRPC server startup with tonic
• Cargo.toml — Added tonic-prost, tonic-prost-build dependencies

Testing

• cargo build passes
• cargo test passes (10 tests)
• cargo clippy -- -D warnings passes
• cargo fmt -- --check passes

How to verify

cargo build
cargo test
cargo clippy -- -D warnings
cargo fmt -- --check

Related issues

Refs #76