add python grpc client

[kacy]

summary

adds the official python client library for ember's grpc interface. thin wrapper
over generated protobuf stubs with an idiomatic python API covering all command
groups: strings, lists, hashes, sets, sorted sets, vectors, and server commands.

key points:

• EmberClient class with context manager support
• password auth via grpc metadata headers
• vector ops accept list[float] — protobuf sends packed IEEE 754 bytes (no string parsing)
• generated proto stubs committed with post-processed import fix for package compatibility
• pyproject.toml with grpcio/protobuf deps and pytest dev dependency

what was tested

• 7 unit tests covering client construction, context manager, auth metadata, and
  metric enum mapping — all passing
• verified proto import fix works correctly (generated code references ember.proto.ember.v1
  instead of bare ember.v1 which conflicts with the package name)
• makefile proto-gen target regenerates stubs and applies the import fix automatically

design considerations

• the generated grpc stubs produce from ember.v1 import ember_pb2 which conflicts
  with the ember package namespace. the makefile post-processes this to
  from ember.proto.ember.v1 import ember_pb2 via sed after codegen. this is a
  common pattern for python grpc projects with package name collisions.
• no numpy dependency — vectors are plain list[float]. users can pass numpy arrays
  since protobuf accepts any iterable of floats.
• tests use unittest.mock to verify client behavior without requiring a running server.