Inmemory storage implementation

[SK1PPR]

Summary

• Implements MemoryStorage for the StorageEngine trait using a lock-protected BTreeMap
• Adds in-memory checkpoint save/restore support
• Wires StorageType::InMemory through API initialization and snapshot restore
• Adds storage, API, and HTTP smoke coverage for in-memory behavior

Closes #36.

Testing

• cargo fmt
• cargo test -p storage
• cargo test -p api
• cargo test -p http
• cargo test -p grpc
• cargo test
• docker build -t vortexdb:inmemory-smoke .
• Dockerized HTTP smoke test with STORAGE_TYPE=inmemory:
  • GET /health
  • POST /points
  • GET /points/{id}
  • POST /points/search
  • DELETE /points/{id}
  • GET /points/{id} returns 404 after delete

[Arshdeep54]

LGTM