[ResourceApp][Backend][Feat] Implement group membership endpoints

[saknarajapakshe]

Overview

This PR completes the Group Membership implementation for the Resource bounded context. It adds full DDD-style flow for /api/groups/:id/users endpoints (GET/POST/DELETE) and enforces atomic DB operations with explicit transactions in repository methods.

• GET /api/groups/:id/users
• POST /api/groups/:id/users
• DELETE /api/groups/:id/users/:userId

What changed

• cmd/server/main.go: route registration for group membership
• internal/group/handlers.go: handlers + validation + status mapping
• internal/group/services.go: service methods
• internal/group/repository.go: repository methods with transactions (atomic operations)

Transactions

• POST /groups/:id/users: one transaction → verify group + users + no existing mapping → insert membership rows
• DELETE /groups/:id/users/:userId: one transaction delete membership row (in user_groups table) with check

How to test locally

1. ensure database is ready (MySQL and resource_app DB).
2. apply migrations
3. run tests:
   go test ./...
4. run server:
   go run ./cmd/server/main.go
5. Test endpoints
   GET /api/groups/:groupId/users
POST /api/groups/:groupId/users
DELETE /api/groups/:groupId/users/:userId

realated to #71
closes #78

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Resource application's backend by completing the group membership functionality within the Resource bounded context. It establishes a robust, DDD-style architecture for managing groups and their associated users, ensuring that all interactions are handled with proper validation, error management, and transactional integrity.

Highlights

• Group Membership Endpoints: Implemented full CRUD operations for groups and new endpoints for managing group memberships: GET /api/groups/:id/users, POST /api/groups/:id/users, and DELETE /api/groups/:id/users/:userId.
• Atomic Database Operations: Ensured data integrity by enforcing atomic database operations with explicit GORM transactions in repository methods, particularly for adding and deleting group memberships.
• New Group Module: Introduced a new 'group' module with dedicated handlers, services, repository, and models to manage group entities and their relationships with users.

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[gemini-code-assist]

Code Review

This pull request introduces a new 'group' feature, including models, repository, service, and API handlers for managing groups and their user memberships. The changes involve adding new database tables (groups and user_groups), integrating them with GORM, and exposing a comprehensive set of REST API endpoints for group CRUD operations and membership management. Review comments suggest improvements such as enhancing security by not exposing raw error messages to clients, ensuring transactional integrity in the RemoveUserFromGroup method to prevent race conditions, adding a unique database constraint for user_groups to enforce data integrity, standardizing error messages for invalid IDs, refactoring anonymous request structs into named types for better maintainability, and correcting code style for struct field alignment.

[ginaxu1]

in both #72 and here you are introducing changes in same branch 71-feat-usergroup-endpoints, so there are duplicate changes

please merge #72 first, then in this branch git rebase origin/main to update correctly