feat: add updateMember mutation for updating member information

[knobby-tires]

Description

Implements the updateMember mutation to allow updating member information as requested in #114.

Problem

"Currently a mutation exists to insert a new member but nothing exists to update the data of a specific member."

Solution

• Added UpdateMemberInput struct with optional fields for partial updates
• Added updateMember mutation using COALESCE for null-safe updates

Testing

Tested locally with GraphQL playground:

• Created a test member
• Updated individual fields (name, email, year)
• Verified unmodified fields remain unchanged

Closes #114

[hrideshmg]

Looks good to me, thank you for your contribution!