feat: add GraphQL queries and mutations for persisting member roles

[swayam-agrahari]

Overview

Adds backend support for persisting Discord member roles so that the AMD bot can restore them when a user rejoins the server.

Changes

• Introduced MemberRoles storage for saving role IDs associated with a Discord user.
• Added GraphQL operations:
  • save_member_roles mutation to store roles when a user leaves.
  • member_roles query to retrieve stored roles when a user rejoins.

These endpoints are protected with AuthGuard guards to ensure they are only accessible to the bot.

Related PR

This PR supports the role persistence feature implemented in amd:

➡ Main PR

[Copilot]

Pull request overview

Adds backend persistence for Discord member roles so the AMD bot can store role IDs when users leave and restore them when they rejoin.

Changes:

• Added MemberRoles DB table (migration) storing discord_id → roles[].
• Added GraphQL query to fetch persisted roles for a given Discord user.
• Added GraphQL mutation to upsert persisted roles for a given Discord user.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/models/member.rs	Introduces MemberRolesResponse GraphQL response type for role lookups.
src/graphql/queries/member_queries.rs	Adds member_roles query that reads role arrays from MemberRoles.
src/graphql/mutations/member_mutations.rs	Adds save_member_roles mutation that upserts role arrays into MemberRoles.
migrations/20260310140619_create_member_roles.sql	Creates the MemberRoles table with discord_id PK and TEXT[] roles column.

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[hrideshmg]

lgtm, thanks