[Backend] Guild Member Leave Event Handling

[gabito1451]

Independence Note: Focus on the cleanup logic. Mock the authorization check.

Context:
When a member leaves a guild, we need to gracefully remove their membership record and log the departure.

Objective:
Implement a clean DELETE /guilds/:id/members/me endpoint.

Acceptance Criteria:

• Ensure the user is currently a member of the guild.
• Delete the GuildMembership record in Prisma.
• Append a 'MEMBER_LEFT' event to the GuildActivityLog.
• Handle errors where the user tries to leave a guild they aren't in.

Technical Pointers:

• Add a guard to prevent the last remaining Admin from leaving without promoting someone else first.

[estyemma]

@estyemma has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hello Maintainers,
I’m a full-stack and smart contract developer with experience building responsive frontend interfaces, scalable backend systems, and secure blockchain applications.
I’m eager to learn, improve, and collaborate with the community while contributing meaningful value. I would appreciate the opportunity to work on this task.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @estyemma to this issue.