Fix gRPC crash when memo content contains invalid UTF-8

[ManakRaj-7]

Fixes #5392

Problem

ListMemos can fail with: grpc: error while marshaling: string field contains invalid UTF-8 when stored memo content or derived snippet contains invalid UTF-8 data.

This causes the entire API response to fail during protobuf marshaling.

Solution

Add defensive UTF-8 validation in convertMemoFromStore before constructing protobuf responses.

If invalid UTF-8 is detected:

• The request fails fast with a clear INTERNAL error
• The affected memo UID is included in the error message
• No user data is modified

Scope

• Backend only
• No API contract changes
• No data mutation
• No frontend changes

Notes

This is a diagnostic-first fix to improve API stability and observability.