fix(api): return AlreadyExists error for duplicate memo IDs (AIP-133)

Ensure AIP-133 compliance by returning codes.AlreadyExists when
a duplicate memo ID is provided, rather than a generic database
error.

Handles unique constraint violations across all database drivers:
- SQLite: "UNIQUE constraint failed"
- PostgreSQL: "duplicate key"
- MySQL: "Duplicate entry"

This provides a clear, standardized error response per Google's
API design guidelines (AIP-133 for Create methods).

server/router/api/v1/memo_service.go

@@ -70,6 +70,13 @@ func (s *APIV1Service) CreateMemo(ctx context.Context, request *v1pb.CreateMemoR
 
 	memo, err := s.Store.CreateMemo(ctx, create)
 	if err != nil {
+		// Check for unique constraint violation (AIP-133 compliance)
+		errMsg := err.Error()
+		if strings.Contains(errMsg, "UNIQUE constraint failed") ||
+		   strings.Contains(errMsg, "duplicate key") ||
+		   strings.Contains(errMsg, "Duplicate entry") {
+			return nil, status.Errorf(codes.AlreadyExists, "memo with ID %q already exists", memoUID)
+		}
 		return nil, err
 	}