[M3] gRPC/REST query API server

[Fosurero]

Description

Expose the query optimizer via network APIs so external services and AI agents can query indexed NEAR data remotely.

Tasks

• tonic gRPC server with QueryService RPC
• REST API with axum (JSON responses)
• Endpoints: /query, /query/temporal, /stats
• Authentication (API key)
• Rate limiting
• OpenAPI / Protobuf schema

Acceptance Criteria

• grpcurl and curl can query the running server
• Query latency overhead < 5 ms over in-process calls