Migrate from deprecated grpc-middleware chain functions to standard grpc package

[enarha]

The grpc_middleware.WithUnaryServerChain and grpc_middleware.WithStreamServerChain functions from github.com/grpc-ecosystem/go-grpc-middleware are deprecated.
Location: cmd/api/main.go (lines 256, 265)
Required changes:
Replace grpc_middleware.WithUnaryServerChain(...) with grpc.ChainUnaryInterceptor(...)
Replace grpc_middleware.WithStreamServerChain(...) with grpc.ChainStreamInterceptor(...)
Remove the unused grpc_middleware import
These are drop-in replacements with identical signatures. The functions were upstreamed to the standard google.golang.org/grpc package in v1.28.0.